Hi Mariah,
Well, there are a couple of negative aspects to my job (although they are far outweighed by the positive ones!). One is sometimes there are people (usually at meetings) that I have to interact with that I don't like. Of course, that is likely to happen in any job. Usually, I try to make sure these aren't people that I'll be working with routinely.
Another aspect is meetings. Not scientific conferences, which are generally enjoyable, but group meetings which are sometimes inefficient and interrupt other things I might want to be working on. These are usually necessary and not too often disagreeable, but every so often one really drags on and the discussion is about something that I'm only marginally involved with.
For my job I write scientific papers as well as proposals asking agencies like NASA and NSF for money to fund my research. While writing these papers and proposals is not bad (and sometimes enjoyable - although getting started is sometimes tough), it can be tough to read a referee report criticizing my scientific paper or the explanation for why my proposal wasn't selected for funding. Usually these criticisms and rejections are worded such that it isn't too upsetting, but there are occasions where the reports are written in a rude manner (I'm mostly thinking of responses to scientific papers). It helps to have a thick skin, confidence in the quality of your work, and not to take things too personally.
There are probably other aspects, but those are the ones that I can think of off the top of my head.
Best,
Christina