how does ones mood affect a relationship?

Depends on the mood, the reason, the frequency, duration and how the other partner deals with it.
I can get in a bit of a mood when I'm (really) hungry, if my bloodsugar dips a bit. But doesn't mean it has to affect my partner, as long as I don't bother him with it ...
The other day someone came up with what he'd do if I was in a bad mood, must say it was a very interesting approach. So it also depends on how the other person deals with it.

I agree with Crystal.

If someone is always in a crappy mood and not willing to address whatever is causing it, I don't see the relationship lasting very long.

I agree compatability within the relationship can affect mood. That is why similar personality type (introvert) and similar sense of humor (just weird) is so important to me in a mate.

No likes to be in a bad mood, and no one likes to deal with another who is in a bad mood. So surrounding oneself with others and circumstances that prevent that (as much as is possible) can only help.

Some people are also less moody than others. Younger folk in my opinion tend to be a little moodier. It can be a maturity issue.

it depends upon the personality

for me, I have a strong empathetic nature. So I seem to 'absorb' the moods and emotions around me which can be very draining if those moods are negative or very invigorating when those moods are positive.

I would never last in a relationship with a gossip or pessimist or conspiracist,, it would be too draining for me

I can deal with more natural patterns of ups and downs with one person though.

I was in a relationship with a woman that had a mood/mental disorder. I'm very patient and even tempered. But after a year or so, I'd had more than enough.
I feel that some people think that by being in a foul, angry mood, they are in better control of life around them.

It depends on the mood.

It is one thing to be in the mood for love.
It is another thing, based on my observation, to have PMS.

100 % agree

am similar to you

These days with the Scottish weather and all the rain, affects my mood a bit. Doing Roofing work, it's impossible to open up one part of a roof, strip it back, put OS boards down and felt the roof to get it watertight and put down the counters also, while it's bucketing down with rain. Still I have to pay my workers, as it's not their fault it's raining! Anyway, that's my gripe!

I've dated one or two rather moody women in my time. I prefer the ones that actually tell me why they are in a mood with me. Said to an ex that was like that recently that at least she communicated.

I agree, when someone is moody and silent, it's just annoying and frustrating. If they actually speak up and tell you what's going on, at least you can attempt to fix the problem, if possible.

Really