I never really have a good answer, unless I feel like going way in depth into things, which I don't. So from here on out I'm going to give the real reason, the first thing that pops into my head when someone asks me why I haven't proposed yet.
It's because I'm scared.
Oh, but I'm not scared of commitment or anything like that. I'm scared because damn near every newlywed girl I know gets back from her honeymoon and BAM she chops all her hair off. Congratulations buddy, I now pronounce you husband and husband. Betcha can't wait til she gets huge and stops having sex with you too.
Also... Why can't anyone ever say something good about their spouse? You ask them about their marriage and they give you this monotone, rehearsed response of "Oh its great... best thing that ever happened to me... I love _____ so much more each and every day..." But when you actually hear them talk about their wife/husband they make it sound like they're in a concentration camp, just begging for someone to put them out of their sad, miserable existence. Makes me really look forward to it. People talk about marriage like it's the seventh circle of hell on earth, then wonder why I haven't popped the question. Shitheads.