Why do relationships all tend to suck eventually?

They always start out fun, romantic and exciting but always end up being dull and boring.
If the only way to keep things going is to not fall into a routine, then how do you do that with someone that's supposed to be a part of your every day life.

I've always thought relationships are like a cave you can sort of seek out comfort, and provides a sense of a home, and vice versa.

Recently, I think I've become more and more lost. I know that communication is the key and all that, and I know that it's unreasonable to expect someone to read your mind. But surely there are times that you just don't want to talk about somethings, or that you're upset about somethings and you want the other person to notice? But maybe because everything is so "routine" that they miss out on some things. Similarly, maybe because they're part of your every day life that you take them for granted or them you. Maybe it's because we start taking them for granted that everything starts to lose its taste and flavor.

I think in a way I've lost so much of myself in the last 7 years that I forget what it's like to be me and only me. Maybe I should be a lot more independent and rational. The idea of being single is so alluring...