Well, we like meeting new people, we probably would have to charge the roommate less and there are different social obligations for a roommate. (For example a guest that stays for a few days is unlikely to leave a mess in the kitchen). In addition any little annoying habits don't have enough time to become tiresome.

Lastly, we can have the room available for our friends and family when they come and visit (which is the point of having a guest room in the first place). My parent stay there (for free, of course) about 2 nights a month.