There are a lot of reasons but it just boils down to "when I've quit, life gets better, when I smoke, life gets worse."
I've done that entire "maybe if I solve X mental issue I can smoke again" strategy, and when I quit, that issue gets solved, and when I start up again because the issue was solved, the issue comes back.
Some issues have been:
I'm not comfortable with myself
I feel isolated
I'm bad with money
I don't like my job
I feel depressed
Maybe there's some deep underlying issues I could try to therapy my way out of, but the obvious common denominator seems to be weed.
It's hard to explain it, but those are my observations.