Mid/late 30's is often when reality really starts to hit, in whatever way. In my case depression.
From like age 14 I always did whatever the hell I wanted. All that mattered to me was fun, enjoyment, gratification. I did everything excessively for like 20 years. Drugs, video games, porn, sex, gambling, internet, whatever the brain demanded. It actually worked out well for me: Physically healthy, no financial worries, decent job, hot wife. So no reason to stop living that life style.
But as it is the human brain keeps asking for more and eventually nothing gets enough Dopamine flowing (except really hard drugs I guess which I thankfully never started with) and you fall into a hole. Nothing is fun anymore, nothing has meaning. Just emptiness.
I know this isn't really news but a lot of people probably think in that situation "Can't happen to me" since you're always happy - until you're not.
If you're in a similar situation give you're brain a break every now and then. It's fine and healthy to just have boring or bad days sometimes and not always fun and enjoyment. Chasing highs never works out well in the end.