Why did Lea become a heroin addict?

I really enjoyed the film (to the extent that you can enjoy films like these), but I was left puzzled as to why Lea became addicted to heroin. The only real explenation I could see was to do with 1) the traumas from her upbringing, which would of course "explain" it, and 2) Henrik's suggestion that she needed something more. This seems slightly contradictory to me though. Henrik at first is portrayed as a kind of saviour for Lea, at the same time as he seems to get her into drugs. I get the feeling there's some kind of missing link here, and I think it would have improved the film if Olin had spent more time exploring and explaining Leas personal limits, and how a life with an alcoholic mother and abusive step-dad can move those limits.
I think this in a way is a story about Lea's mother just as much as it is about Lea herself, or so it seems to me.