People usually don't go into it thinking they'll become addicts. Many think it won't happen to them, and I guess some just don't think about it at all
People use because it feels damn good. Some have a fine life and do it just for fun. Some can keep it as just an occasional fun, but eventually one will decide to indulge just a bit more, maybe from once a month to once a week, then indulge a bit more, and eventually you get to the point where you've indulged frequently enough you need to use every day just so you don't feel like shit
At this point, some people quit, and some people will keep using to not feel shitty and to feel damn good again
However, some people do use it as a crutch. Some use it to cope with certain things in their life. They're depressed because they were dumped/loss of a family member/whatever, and then begin to associate happiness with being high and sadness with sobriety, and it's hard to escape that mentality
Some people also self medicate too. For example, I know some bipolar people tend to use to cope with the lows
Point is, addiction is far from a simple matter. No one goes into drugs wanting to become an addict, most think it's something that happens to everyone else, but they have enough willpower to avoid it.
And there's no nice clear line between addicted and not addicted. It's a gradual process, and there's usually not a crystal clear moment when someone goes from just using/abusing to addicted, it's a gradual process. Like I said, it's a very complicated thing
Sorry if I ranted/didn't make sense. Tried Adderall for the 2nd or 3rd time (and had a Call of Duty marathon)