Why relationships fail...

Here's a graph of affection vs time.

T0 is when two people meet. T1 is sometime into the future when the relationship begins (this varies from couple to couple - it's known to coincide with T0 on occasion). T2 is when things are at their relative best. And T3? We'll get to T3.

Most relationships, not just the romantic, follow this curve. Acquaintance, followed by friendship peaking at T2. Then you notice some stuff you don't really like about a person. They can't take jokes on certain topics. Or they occasionally pretend to have read a book they haven't (annoys the shit out of me). Or their (surprisingly well disguised) tummies are bulgy. It could be anything.

And so your affection dips a little. It drops from Y2 to Y3. From there it remains relatively constant. There are minor ups and downs, but it stays pretty much at Y3. Friends can stay at Y3 for years, decades even.

Lovers can't. And so T3 is the beginning of the end.

The difference between Y2 and Y3 (for you engineers complaining about the lack of a scale on the graph) is close to negligible. On a long enough timeline, it would be imperceptible. The problem isn't how much smaller Y3 is than Y2. The problem is simply that it is smaller.

People expect love to be a constantly increasing function stretching out to infinity. Or at the most pessimistic, they expect it to peak and stay at that peak. The magnitude of Y3 ceases to matter. Y2 sloping down to Y3 tells them that they do not love as much as they once did and that scares them. Television, books and movies bombard us with those images of a constant, enduring love.

And so that little dip, that comparison to what once was, prompts us to end something warm and real and propels us back into the world in search of that illusive, elusive infinity.