Take the Netherlands. From what I know, we pay more to the EU than we receive in subsidies and stuff, and hopefully trade is making up for that (who knows).
Take Spain. If I remember correctly, they're getting more from the EU than they pay, so for them it's a definite win.
So if the leaders from these countries go and negotiate, what's fair? Should we share all our wealth next month with the rest of Europe? Sounds unfair towards us. Should we keep it all and not invest in the EU? Also not a great idea.
Every penny we pay will make us more unhappy; yet every penny we don't share will make the other party more unhappy. I'm not sure there always are good outcomes. (This is probably one of the reasons why governments and politicians are fairly universally hated.)
A similar example could be constructed with the refugees issue. Some countries are being overrun; others only receive as many as those front-line countries send through. The front-line countries want to send as many as possible; we want to get as few as possible. Every extra refugee makes our representatives less happy about the deal and every refugee less makes them less happy about the deal.