Some people are ideologically opposed on the basis that the majority of global citizens still haven't been offered a first vaccine.
Some feel they have already got protection against serious or lifethreatening illness and don't mind catching it, much like they do with a cold or other virus.
Some don't want to go through the side effects again.
Some are sick of "being told what to do" and this one way they can rebel.
Some are waiting a little while longer as have antibodies from both the vaccine and the infection.
I have had a third dose. I was unsure if I would be offered it and was very anxious that I didn't have antibodies (autoimmune disease and neutropenic) and am back teaching in small cramped spaces on a campus with ten thousand students, 20 % of whom are unvaccinated plus staff. Our campus has done away with masks and social distancing. I arranged an antibodies test so I knew what I was dealing with. I was offered a 3rd jab with an hour's notice.
My antibodies test showed almost 2500 units, very high level, especially given that I was vaccinated in January. It arrived a couple of hours after my 3rd vaccine. I think I would have delayed it a few months had I known. I might now need another dose in Feb, as I think the infection rate will still be high whereas if I'd left it another couple of months I think that the case rate will be lower in March or April and I probably wouldn't feel the need to have a fourth dose in the space of a year.
I'd prefer to just have one a year eventually. I do get bad side effects and I am a little worried about the possibility of long term implications even though this is not founded on any evidence or science. Just my own anxiety.