This is a tricky one. Here is my guess:
Basically I think all autistics are the most happy when they are allowed to be their autistic selves - that is, being their true selves. The world will seem very bright when you are in your "true autistic mode" (think of your most favourite interest).
This world is, however, not built for autistic thinking. Autistic thinking is aimed at thinking about surroundings - e.g. how do things work, how can we invent new things, caring a lot for others, discovering the world etc. Basically everything that is not really related to your own personal life. This world requires non-autistic/neurotypical thinking (to survive), which is thinking aimed at oneself - what are my needs, how can I fulfill them, what persons should I keep in touch with in order to keep a good social network, what problems do I have that need to be solved etc.
So autistic people are trapped in this dilemma between being their true selves and thinking neurotypically. On the one hand, if you are never your true self, you won't feel good, but on the other, if you don't think neurotypically, you won't fulfill your needs, and then you won't feel good either.
It becomes a matter of compromising the two sides.
When you act more neurotypically you might experience euphoria because of increased self-confidence. This world rewards neurotypical thinking, that is, thinking aimed at yourself by increased self-confidence. When you do something good for yourself your belief in yourself typically increases (I believe this is what you've experienced).
But at the same time, when you act neurotypically, you compromise your true self (to some degree). My experience is that the world becomes way too superficial if one keeps doing that for too long at a time. You have to be yourself once in a while (e.g. alone-time/spending time with other autistic people).
When people on this forum say they sometimes forget to eat, it's (most likely) because they have been thinking autistic too much that day.
Other autistic people might be depressed because they've been forced/or forced themselves to act too neurotypically for too long.