I run a lot and it used to be like that, but now I'm desensitized so that sometimes even after I run a lot I still feel down. It's also hard if you're injured or tired.

I don't see why you're downvoted though: runner's high does give me something to look forward to / some way to relax so I wouldn't need to do something like drink excessively.

Yes, though in my case I would say its more a slight mood lift or relaxation than a “high”, but the effects of exercise and alcohol are very similar and not cumulative, so if I’ve already done one its kind of redundant to do the other.