I'm far from an expert on relationships, but I would imagine it's like having your favorite food every day for the rest of your life.

Sure, the first week is awesome. But the 50th week? The 500th? Eventually your taste buds are dulled to the sensation of steak/pizza/whateverthefuckveganseat.

Those first few relationships you have are like that. Once you have too many of them, they aren't special anymore, and if the relationship isn't special, the partner isn't either, and there goes the ability to connect and be intimate with them.