Idk, I've never been happier than the six months I was dating this girl who's sex drive was almost as high as mine. Never been able to get there on my own and the other aspects of my life have been way better since then. Sadly her mental health was a mess and that relationship didn't last.

happiness comes from within

In my experience, having a constant companion has a positive feedback loop. People you can continuously interact with - joking, catching up, eating together, helping one another out, just Netflix'n'Chilling... it's reaffirming.

But it is a loop. You don't just wake up happy forever. There's ups and downs. There's psychical and emotional adjustments. You're not immune to despair. You just have someone you can be glum around who - ideally - fills you in on the lows and rides with you for the highs.

If you've got a bunch of mental baggage going into a relationship, your partner (ideally) helps you unpack that shit and dispose of it. Or, at least, shows you their own baggage, so you know you're not alone. It doesn't just go away instantly, but over time you can put it behind you precisely because you've got someone else in your life affirming your own worth.