Oh, my! A question I can seriously answer!

Source: studying to major in psychology!

So, the answer is: oxytocin! Among other nuerohormones, as a few other users have so kindly mentioned.

Those of you who don't know - oxytocin is the love hormone. It creates emotional bonds, it can increase the rate your body will heal, but most especially it is the bond between a mother and child at birth. I believe it also stimulates the production of breast milk. But that's unimportant. When you orgasm with another person, your brain and their brain releases oxytocin (among other things but I'll get to that) and it's intimate, it's nice.

Assuming you enjoy that person, your brain should also release dopamine, which is both the happiness neurohormone, as well as "the relief hormone" - I say this because, for example, if you're a smoker hankering for a cig, and you finally smoke it, your body will release dopamine in response, causing you to feel satisfied. So sex - you and your partner have orgasmed, you're tired, but your mood is elevated, you're attracted to them, you probably feel satisfied and not to mention the release of epinephrine, which is similar to adrenaline (might be wrong, may be norepinephrine - i'm really tired).

As to why you'd feel guilty after masturbating solo - you mentioned you're a /r/nofap member. Perhaps it's because you feel as though you failed that? The sense of guilt is subjective, so all we/you could answer is why it applies to you alone, or why it would apply to one of us personally. The answer varies person to person.

TL;DR you're happy after sex with a party because your brain releases chemicals to make you feel that way. And only you can answer why you would feel guilty post-jerk.

Sorry for rambling! It's very late, I'm very tired, and having the answer to this is exciting! Science is cool.