As these questions hint, my feelings vary on how safe I evaluate the situation to be for myself. I'm not likely to have a fun if I feel unsafe. Why do I evaluate a situation as unsafe sometimes? It's because alcohol lowers inhibitions and can lead to people saying and doing things they might not otherwise. When I'm in a space with people I know, people I can trust to be cool and reasonable even after a few drinks, I feel safe and I can have a drink or two.
In other occasions, I don't feel safe. I can't see the future, and I don't know how strangers will behave when drunk. History tells me that sometimes they'll act badly and sometimes they won't. It's not fun for me to gamble on a hope bad things won't happen this time, so I tend to skip parties with drinking on these occasions.
TLDR: Sometimes drunk people do stupid things. I don't like gambling that this won't be one of them.