To back up what BNF said I don't think it is as much a compulsion as it is dedication. A lot of people might see them in the same light as stupid jocks in some fraternity somewhere, but what some people don't realize is how organized and structured their life is. They don't go out and toss around a few weights any way they want, go party all night long, and then roll out of bed the next morning huge. They are careful with how they plan every meal, every workout, and when they sleep. Sometimes they plan what they want to do months or years in advance. The stupid jock couldn't do it; he just doesn't have what it takes to maintain what needs to be done for that long. You need to keep at it, there is no "I don't feel like it today" or "I want to take a month or two off". Even people that take steroids have to keep at it or it wont have the effect they need. As soon as somebody thinks like that they have already half lost the battle. They don't have any mental condition, not because of body building anyhow. I would say that only similarity between then and somebody that is truly obsessive-compulsive is that they are highly organized.
Also, whenever you gain muscle you get stronger. You can't get strong without putting on more muscle and you can't put on more muscle without getting stronger. There might be a difference between top body builders and power lifters in what they can do and how they look but they are both strong. That isn't to say they are all perfectly healthy however, because some of them aren’t. A lot of that has to do with steroids among other things they do to themselves, but they are strong. Otherwise people that do nothing but push around pencils all day could have 20-inch biceps.