When I'm down about social phobia, I think "ugh, I hate being such a wimp at standing up for myself". Other times, like today, I think "I'm learning to stand up for myself, and as a bonus I have more tact than others might". In general though, especially with new people... yeah.. I definitely have trouble giving my true opinions. Like somebody already typed, I'd feel bad about hurting their feelings or perhaps starting off a potential relationship on the wrong foot.
Over the summer (something I think about whenever I think of an example of when I expressed my own opinion) when the latest and last harry potter book came out, a Vet at the Veterinary Clinic I help out at and I were talking about the book briefly. (All my conversations with "new" people are brief lol). I said "I'm loving it so far. I just find that its weird, how certain people read it so fast. A friend of mine is almost finished. I just don't see the point of waiting for a book to come out for 2 years and then reading it all in a day or two!". Right after I said this, she said "I finished it last night" (which was the day it came out). She had been heading towards another room which made things easier for the conversation to end, and I said "you're crazy!" in a friendly way. I then thought "oh my gosh, I hope she doesn't think I'm being critical towards her". I realize that she probably doesn't remember this, but even so. I guess all in all I have to like that having social phobia has made me more sensitive to others' feelings.