Sharing is great. I really didn't want to share, like you, got the letter that said I'd be sharing and was really pissed off and unsure if I'd made the right decision about uni.
I now can't make a high enough recommendation that you share in your first year. It may seem daunting at first, but it really is a great and life shaping experience. Coming to university is less about spending X hours a week studying the intricacies of some arcane subject and more about learning how to live. Studying is just an excuse to spend three or four years in a semi-sheltered environment learning who you are and how you want to live your life. The college will do its best to put you with someone who you'll get on with, and remember, they've been doing this since 1832, so they know what they're doing! Obviously there are some mismatches, but the likelyhood is very low, and I know of only 2 roommate pairs who properly fell out with one another out of everybody in castle. Of course more than that decided that they didn't want to continue sharing with one another, but parted ways amicably.
Hatfield is a great college (much as I am loathe to admit it), and I don't think it would be a good idea to be put off by the fact that you *might* have to share a room for part of your first year. Getting an offer is a great achievement, and turning it down because you're unsure of such a little thing is ridiculous. (After all, the amount of time you actually spend in your room is negligible).