I remember reading some good advice about painting that I think could apply to writing. Maybe you've heard it before...
On a blazing hot summer day, an art teacher told his class to paint a snowy winter scene. When the students complained that they couldn't get in a wintery mood on such a hot day, the teacher told them that if they were ever going to be working artists, they must learn to channel their past emotions into current projects. By seeking out their own inner inspiration, rather than waiting for inspiration to strike, they would always find a way to create. After all, if an artist mastered a winter scene during the summer, imagine how effortless it would be to paint a winter scene during the winter.
Applying this to my writing, I tend to take on my characters' moods and feelings, not the other way around. If I'm feeling angry, I don't need to write an angry scene. I write whatever comes next in my story, and bring out from myself the emotions my characters need. I find that almost right away, I am writing not as myself at all, but as my character. It's incredible and thrilling, and sometimes I don't even know it's happening until I'm interrupted for some reason. I don't need to be in the same mood as my character, because it feels like I've become my character... moods and all. I'm sure other writers can relate. For me, becoming my characters is one of the many reasons why I love and adore writing.
What about you? Obviously it's easier to write a sad scene if you're already sad, but can you overcome it and write what needs to be written? Even if it's a joyous wedding scene or a powerful first kiss? Is it problematic if the way you feel doesn't match your characters' emotions?