I've never noticed, until recently, how very easy it is to go from extremely happy to extremely sad, all within the span of a day, or maybe two. Sometimes, within a minute or two. It seems to happen more than usual to me nowadays.
I know this guy who told me that every girl he's ever dated is completely bipolar - that they go from extremely happy and wonderful to completely depressed, and they're too unpredictable for him. He says it happens every time.
My advice - welcome to the world of women.
We're really not THAT bad though. You'll just need to get used to some mood swings. The extra estrogen makes us a little bit bipolar sometimes. Especially for about three or four days of the month - we're especially bipolar during that time. So be warned. You'll never meet a woman with a completely steady flow of emotion. We're just not designed that way. - that being said, at least we don't go punching holes in walls when we're angry (I've heard this was done by... 5 guys so far) - girls just tell you we're angry. so HA!
Relationship wise - I think it's pretty normal for all of us to have good days and bad days - for certain people to deal with stress better than others - while some of us are particularly bombarded with things one day, and completely free the next. I think girls are emotional creatures - and we don't hide it very well (most of us anyways). Guys are emotional too - but they bottle it up inside most of the time (though not all of them) and then punch a hole in the wall. So what's better? Maybe... neither. You just have to deal with those tough times in life, and love the person anyways. Best advice I can give on that topic.