I think a lot of people turn into dickheads after they start smoking weed because it's a new thing for them, and they have often just gotten involved in a lifestyle that they have half fantasized about. This is an atrocious paragraph, let me try again in a different fashion, as a kind of example.
I started smoking weed almost a year ago. I tried it a couple of times and then got into it a bit too much, 2 or 3 times a week for a month or so. Before this, I had always seen it as an almost taboo, nothing too bad, but generally frowned upon. As a result, when I started smoking, not only was I enjoying it, but I had added an aspect to my life that I had seen to be rebellious and for a different group of people, and I felt a little bit cool doing it. Being an extremely intelligent and profoundly wise person (and with a little help from my girlfriend), I realised I was being a bit of a dick, and that fun as it is, smoking weed is not cool.
I now smoke weed occasionally, maybe once every couple of weeks, and enjoy it simply for itself and hanging around with friends, and it is no cooler than drinking alcohol or wearing speedos.
Other people I've seen never quite let go of this idea that it's a cool thing, and they change their personality to suit their views, rather than the inverse.