Meth is super cheap. Can get high all day on a single toke ($2ish). Injecting it can last 2-3 days. Weeks on end of not sleeping causes this. I met a guy that was smoking $200 a day of Meth, he said he 'boosts' (shoplifts) to provide for his addiction.
In terms of 'how it happens', it's the chemical. How easy is it to quit caffeine or cigs? Multiply that by 10 or more and you're in the realm of Hard Drugs now. Can happen by accident… you're just curious, or your faggot dealer laced your Weed with it or convinces you to try harder drugs by offering it for free at first. Same way your buddy offered you the first Cigarette.