A phrase constantly repeated in my anthropology class comes to mind: "to survive and reproduce". You have the survival part down, but you are missing the other half. I think anyone without kids is at high risk to feel empty as they get older (I say this as someone without kids).

Maybe adopt a dog? I think you can even foster them if you just want to try it out.

Lifting heavy does wonders for my mood. If you can motivate yourself to get to the gym, Starting Strength and 5x5 are good programs.