The study results are somewhat interesting. Dopamine is arguably one of the most complicated to understand neurotransmitters in the brain.
L-dopa increases dopamine in general in the various dopaminergic areas of the brain. This is good for Parkinson's patients because they suffer from both mental and physical problems from their low dopamine levels in particular areas of the brain. Schizophrenics have brains that are practically bathing in dopamine and not in the "pleasure" areas alone; their prefrontal cortex is flooded with it which is where most thought-perception occurs (I think). In fact, one of the symptoms of L-dopa toxicity in Parkinson's patients is hallucinations and psychotic type phenomena-it happens over time as the doses get into the grams/day level. Then they have to switch to carbidopa, etc.
As the article points out, l-dopa isn't really associated with being "happier", which the study probably defined as a euphoria-type happiness since it is easiest to measure on an established scale (I dunno-but that's not really "happiness" anyway).
On the other hand, administration of amphetamines and cocaine cause a bit more of a targeted effect and typically cause an almost giddy euphoria in the majority of study patients given one or the other. They don't do too many of those studies anymore! However, I remember one study from the 70's (academic database) that would be unethical per the APA today. A person was administered IV (!) 10mg of dextroamphetamine (they don't even make IV amphetamines anymore) every hour on the hour and reactions noted. Initially, euphoria, etc. But within 24 hours and 240mg of IV amphetamine given (a freakin ton), he was in pretty much full blown toxic psychosis. It went away after a day or three as the drug left his system and he went to sleep, etc.
The same is true for the opioids and other abuseable drugs, including steroids. Pretty much anything that is rewarding and/or causes that momentary euphoria is associated with the release of dopamine in very specific parts of the brain only. These parts are mainly in the hindbrain (nucleus accumbens, ventral tegmental something-or-other, etc) which is the most primative part of the human brain and which is why rats/cats/dogs/etc respond to amphetamine/cocaine/opioids/etc as well and are often studied. It is also why junkies act like animals; they function on a very primative level in that, as the article alludes to, dopamine is associated with learned behaviors. "Roll over, sit up, etc" = "Get more smack, inject more smack, get more cash, repeat". That try a drug once and you are hooked stuff is generally non-sense; it is a learned behavior. People who are "hooked" from the first use of anything generally have very weak will power or other psychological issues such as depression.
Some drugs like crack (no, I've never used crack before) only give you a few get-out-of-jail-free cards in this respect before you inadvertently acquire a new learned behavior that intensifies with each additional use of the drug so the hole you dig gets deeper and deeper. To pick on crack heads some more, they will often choose crack over food b/c it is much more mentally rewarding and though that next rock only gets them high for 5 minutes, they are immediately looking forward to that next crack rock and how to get it, even after they really can't even get a buzz anymore-the mental drive is just that strong.
The learned behaviors can be a good or a bad thing depending on what you are learning and whether you are using drugs as well. Heroin addicts often inject saline solution when they are out of the drug because the brain is used to an injection followed by a profound euphoria. Dopamine levels actually rise briefly to euphoric levels in anticipation of that euphoria though no post-injection euphoria is experienced. Again, a learned behavior. The ritual of smoking cigarettes (packing them, lighting them, drawing on them, inhale, exhale, etc) is another example.
And for the bodybuilders of course. Steroids have definitely been shown to be associated with psychological cravings and sometimes physical dependence phenomena due to the myriad hormonal changes in the body and the Post cycle depression if present (don't get me wrong; I use them too). This is why some people literally never stop using steroids-30 week cycles and then you are so effed up that you need some test just to function since you have zilch. This applies to oral and injectable steroids, but I think that injectable steroids, relative potencies not being a factor, would be even more associated with psychological dependence than orals for this reason. Some steroids are inherently somewhat euphoric as well (some are dysphoric like good ole M1T).
The learned behavior of the ritual of steroid injections. Wipe the top of the vial with an alcohol wipe before inserting drawing needle, draw 3 cc's of oil with vial upside down (or right side up), remove needle from vial, flick syringe needle up, squeeze out air bubbles, aspirate slightly, take off dull needle, replace with new needle, wipe injection site with a new wipe, don't unsterilize the syringe and needle to be injected (unless you recapped it and put it on a sterile surface), squeeze skin taut if applicable, inject needle somewhat forcefully deep into the muscle, steady pressure on the plunger, remove needle with a pretty fast, smooth, motion, put a sesame street band-aid on if actually needed, repeat daily or weekly. That is a hell of a lot of learned behavior and ritual (necessary), and add to that the expectation of great gains in the gym and you have quite a mentally reinforcing package (esp. when the gains do come rapidly in the gym). Thankfully, steroids aren't exactly as addictive as crack, but it is little wonder that users are often looking forward to that next injection or that next cycle or whatever. Or people who never quit either one long cycle (wrestlers) or cycle after cycle their whole life.
The real question is at what point does it become deleterious to your life and do you stop using them at that point or do you continue on in spite of the effects they might be having on your life or health?
But it isn't all bad. Bodybuilding itself is very rewarding and in a healthy way. You work hard in the gym and you are rewarded for your hard efforts and you don't necessarily have unrealistic expectations as far as gains go (though you probably do if you have dabbled in steroids and seen the results) and you are pleased with the results. And you have less mental stress and are in better physical condition, etc. So this can be quite a healthy, learned behavior. I am not saying don't use steroids (or use them); it is just easier to look at one variable at a time.
I always felt like that when I left the gym anyways, even if I came in with a crappy mood. Probably the endogenous opioid release from the pain of working out ("runner's high"); of course endogenous opioids have a direct affect on the mood but also have a dopaminergic effect as described above.
But the body is good at regulating itself in healthy manner if you aren't taking drugs that try to circumvent your body's built-in protection system. The dopamine rush associated with sex or a good workout or both (LOL) is quick and fleeting because the dopamine transporter quickly mops up the free dopamine molecules which are reabsorbed into the neurons of the brain (ready for round number 2?-LOL!). Cocaine blocks the dopamine transporter from doing its job so the elation is unnaturally prolonged; amphetamine just floods the synapses (spaces between neurons) with dopamine, but the end result is similar. Both drugs exert most of their effects in the dopamine neuron heavy areas of the brain. Think of it as free testosterone versus total testosterone if you want.