I don't know that I was always quiet, but I definitely turned very quiet around the age of 17-18. I may have justified it to myself as being a good thing because I was able to learn to listen very well, and really hear what people were saying, so I derived something positive out of that, but I do think it had a lot to do with feeling unheard myself and feeling like there was no real point in talking because no one listened to me, and no one cared what I thought or took anything I said seriously, and when I did express an opinion it would be rubbished or ridiculed a fair bit. And also with the amount of misunderstandings that seemed to take place in my daily interactions, it felt better to just not talk.
I liked being quiet, though the hardest part was dealing with being looked at like I was weird, and being suspected of being a serial killer for being on my own so much, and not talking or smiling much. What people didn't understand was that I don't have a face that smiles naturally...I've always been a little pensive and on the depressive side, and I accept that about myself. I HATED being told to smile, and it would make me rebel as a teenager and not smile even more, which was not good for me in the long run. I liked being quiet and felt like I was in tune with my God when I was...like the Quakers. It was a positive experience for me, but people thought I must be a serial killer, so, not so comfortable at times...often actually. I got used to feeling quite uncomfortable though, and even learned to relish it as part of my uniqueness. I enjoyed my own company so much, and preferred being alone to being with most people...which no one understood of course because they all think they are so wonderful, when actually they are scary assholes mostly.