Then you, often without consciously trying (if you're me), find you're bringing that energy, positivity, and happiness to your other relationships. It's like everyone gets a little - or a big - pick-me-up from the positivity that a good new relationship can bring. I know, this is idealized. It doesn't always happen and the generic "you" might not be like the actual me. There are all kinds of exception cases and I'm not trying to say this always happens. But it does, sometimes, with some people, and I've seen it.
It's not compersion, the notion that you feel happy because someone you care about is happy. When the Lift happens, you're happier because your partner is paying more attention to you, bringing a more positive self into your relationship, taking extra time and care with you. In the normal course of a relationship I think this happens anyway - people go through cycles and phases and sometimes those are up/positive regardless of the entry of a new person into the relationship web. But when adding a new person to that web leads to this uptick, well, that's The Lift I'm talking about.
Dear readers, does this make sense to you? Have you see this? Experienced something like this? I doubt my experiences are all that unique in this regard but perhaps.
(How do I NOT have a "poly" tag? Sheesh.)