Why doesn't exercise make me happy like it does for everyone else?

It's January and as always, my resolution is: get fit. Not get slim but fit.

Weight (especially around the waist and chest(I'm a guy)) has been a massive problem for me for a couple of years and it's worse now that I'm 16.

My dad and my sister often do yoga and running because they want to get fit(ter) and because it makes them happy.

I've read and heard several times that "exercise make you happy" because it releases endorphins. I don't understand this.

I enjoy squash, I enjoy cycling and I enjoy running (not competitively).

I don't get this happiness from any of those activities and it's worse when it's with other people, it makes me realise how unfit I am.

So I guess I have a few questions:

How do endorphins work?

Why don't they work with me?

Does negative psychology overwhelm / overpower the endorphins?

Does anyone feel the same way?