It's something I always mean to sit down and actually have them explain to me. I'm sort of a stickler for words - not necessarily in the way where I want them to mean the same thing for all people, but in the way that I like to overly justify the way I define a particular word (i.e. "relationship," "love," "marriage," etc.) so I expect other people to have done the same. This very rarely happens, and I usually wind up in the conversation a bit like this: "You mean I spend hours internally justifying the way I use and define words so I can explain my relationship format to you, but you just get to use the same words and not even think about it?"
The way I tend to think about it, "more" is a quantitative phrase (i.e. six blocks is more than three blocks). Regardless of if the blocks are red or blue, made of wood or metal, giant blocks or teeny tiny blocks, you still have three in one set and six in the other. So saying that you love somebody more than somebody else is tricky, because there has to then be a quantitative aspect to love.
It also begs the question of what exactly happens to that love from relationship 1 when relationship 2 starts. Does it erase, blank slate style? From what I've gathered in conversations, it sounds like you have to at least negate the relationship 1 love in some format, or otherwise it calls into question the validity of the love in relationship 2. Shit gets complicated, apparently. And I thought just having to figure out date nights with separate partners was complicated. Imagine if I also had to constantly be reassuring all of them that they were the only ones receiving my somehow finite and quantified love.
I tend to think about love more as a level reached in a video game. It's not a question of who got the highest score once they reach the love level. Fuck, you beat all the previous boss stages. (Allusions here to: "Can you put up with my neurosis?" "Do you get along well with my sense of humor?" "Are we fairly compatible in the bedroom?" "Do I feel I can trust you with things about myself?") It's possible that either one of us might lose interest in the game after reaching the love level, but we still made it to that final level together. Now let's put our initials as something hilarious like ASS on the high score.