It’s a few things. It’s the positive feedback loop of seeing and feeling yourself get bigger and stronger. It’s the rush of good chemicals/hormones/whatever the body releases. It’s the feeling/thought that there is nothing you could be doing that could be more beneficial for your health, and knowing the knock-on effects that has for you long term. Its the pump.
It sounds terrible but I hope the ux doesn’t improve, as it acts as a barrier to entry to a lot of the shit-tier Reddit users.