Why I started doing drugs is a different discussion from why I continue to do drugs.
Why I started is pretty simple, I have always had a love of psychology, attempting to see how my mind works. A friend of mine, who is in training to be a neuroscientist, convinced me that I should try it because it would change the way I think. It has been said that in seeking to understand something, sometimes the best way is to see how it works when seriously altered, so I smoked weed because I wanted to see the change it would create in my perception of the world.
Why do I continue? A number of reasons, some good, some bad. It's one of the most enjoyable activities I can think of, and it makes most other activities enjoyable as well. If you approach it outside of the context of partying and just having a good time, especially with more powerful psychedelics, it can force you to address things you didn't want to address even though you really need to, to see your own life and its context to the world differently, and can be used to make profound life changes. There is an extent to which I feel like it can spiritually connect me, much in the same way that I feel spiritually connected when I practice deep meditation. There are some bad reasons too, I do sometimes use lighter drugs to run away, to either sedate or abstract my problems so as to not have to deal with them for a while. I know that's not good for me, but it could hardly be more human.
The health issues are a concern, certainly, but I consider it within the context of the alchemical psychology movement, that is an equivalent exchange. A trade of something later (some of my life) for something now (a rewarding experience). Sometimes I feel like I get a fair or even great deal in this equation, and sometimes I feel like what I got was not ultimately worth it.
Drugs are like just about anything else, there's some good, and there's some bad. We all make decisions every day, and for those of us willing to look beyond the stigma and the massive campaign of misinformation, we make serious decisions, sometimes good, sometimes bad, related to powerful mind altering chemicals.