How serious people take it doesn't make the game competitive. What makes a game competitive is it actually having a competitive pace and mechanics that don't behave as a handicap to those who already know what they are doing (i.e. aim assist and regenerating health).

Counter-Strike is probably the current epitome of a competitive first-person shooter.

That said, yeah. I never played Halo to prove my worth (or any video game, for that matter). I play them to relax and have fun. If something is causing me to pop a blood vessel (incessant griefers, bullshit mechanics, dysfunctional team) then I'm done. I don't do well in competitive environments because they force me to use tactics and equipment that I feel are cheap/low/not fun for me in order to not get demolished. I don't like being forced into picking up a DMR on a tiny map that the DMR should actually be terrible for.