Phone number or other contact info. Supposing you’re on good terms, they’re not someone you’re interested in romantically or the like (so none of the anxiety from that at least), what’s kept you from asking?
Supposing that something has on occasion, anyway.
Negative responses. The last time I asked someone I thought I was becoming friends with she reacted almost offended and tried to find excuses why she wasn’t able to accept my email address (because I thought asking for her number would be a bit direct). We knew each other for almost two years back then and spent a lot of time walking our dogs together when we met by chance, laughed, exchanged issues we had with our lives etc. I really thought we could just casually meet for our dogs to play, but that reaction was… I don’t know, it did hurt quite a bit. Since then I’ve become even more reserved and try to avoid situations like these.
That’s unfortunate…It really can be hard to read when someone’s open to exchanging contact info of any kind sometimes. Some folks, easy as can be, others you think it may be and then you’ve tripped their brick walls without expecting it.