Sorry, this is gonna get long.
It's because they are constant. I'm a female CS major. I'd estimate that every day in the last ten years (since I got into the technical track at my High School) I got about five reminders that I don't belong, I'm rare, I'm different, etc.
Some were just hilariously stupid (dude asked me if my brother helped me when I fixed his computer, then literally grabbed my crotch), some were very repetitive (I get it, you're very funny. Wow, you thought of asking me "Are you sure you're a woman?". Never heard that before).
Then you get the dudes who are being so edgyTM , start to tell a joke then look at me and say "Oh no, can't do this right now, there's a lady in the room."
The endless stream of people asking "Wow, a woman in CS. That's so unusual.", "Great, that's so unusual.", "Bet you just want to find a boyfriend.".
You get the casual stuff, like the jokes about how female CS majors can't even be looked at. The dudes who think I'm their very own personal nerd girl and are surprised, shocked and angry when I still paint my nails.
A colleague mentioned that his girlfriend installed the Ask toolbar on his computer. "Haha, why is your computer in the kitchen?"
The advertisements that always imply that women aren't interested in tech or good with computers.
My boyfriend just started going back to college (also CS). We were at a party yesterday where a dude shared his brain-vomit about how it's gonna be sooo emasculating that I know more than him ("Well I know who's the guy in this relationship.")
None of these are worth a second thought on their own. No thousand are worth a second thought.
You won't be able to comprehend it because you need to experience it yourself but imagine that your entire life you always hear that you are made wrong. Something is defect in you, you're not a real guy/girl, WTF is wrong with you?
That's the upsetting thing. Again, I still get through my day, most of the time it's fine but every ten thousandth unfunny joke (after you heard it ten times all of them are unfunny) it stings a bit.