I'm going on a trip in May, and I expect it to be great. If it sucks, I might be a bit disappointed, but the vacation is a week, and the time I can look forward to it is several months. If the vacation turns out to be great, I'll feel great about it. Would I feel even greater if I had no expectations? Don't know. I really don't think the delta of getting pleasantly surprised outweighs the months of looking forward to the trip.
Being able to accept what you can't change (e.g. being stuck in traffic) without getting overly frustrated is conducive to happiness, no doubt. I'm not sure what it has to do with expectations; if I expect to be stuck in traffic all morning, that doesn't make it more fun when it actually happens.