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It seems that people are making a distinction between nature versus nurture. One of my psychology classes spent a lot of time trying to break this dichotomy down because it seems to be quite unhelpful. What I took home from the class was that behavior (normal or otherwise) isn't broken down between nature (you have no choice, it is something that happens to you) and nuture (all mental illnesses are character flaws and the people who have them are to blame) but rather nature via nurture.

For example: there are certain alleles (for those of us who aren't biologically inclined: each gene can come in several "flavors" each flavor is a different allele: they do the same thing, but in a slightly different way) anyway, some alleles of certain genes lead to a predisposition to depression. But, you could find 10 people with all the predispositions we've found to depression, and maybe only 7 of them will ever become depressed. The flip side is also true: 10 people could, genetically, have no predispositions toward depression, but life circumstances could cause 2 or 3 to eventually become depressed. (Certain, big, life changing experiences have been found to help precipitate depression and they aren't even all sad: getting married, going to college and having a baby can all "cause" depression).

Outlook, experience and genes all play an interconnected role and influence each other. Experience can cause a change in how/when genes are expressed (anyone who's gotten a tan proves this!). And outlook/personality can change both how someone experiences something and how their bodies respond to the environment (they say that optimists recover after surgery significantly faster than their pessimistic counterparts). And a person's genetics at least partially influences their experiences/outlook (at 5' 2" tall, I'm never going to be an NBA superstar...)