Honestly, you have to keep at it. Our son challenged us a TON on this. We told him when he was 3, he didn't HAVE to wear diapers anymore vs. GET to wear diapers. Well, we put him in underwear and yes, there were a LOT of messes to clean up, but after a week, he was fine - even at night. (He even went so far as to sit on my pillow and pee.) We rewarded him with gold coins or trains - since we don't use food as a reward and we didn't give him sugar until he was 5, so no cookies, etc. Once he saw that his dad and I were serious and a team, he stopped having accidents. It's power struggle and you have to decide who is going to win.
letting the kid decide can cause more issues later with being embarrassed when they are still wearing pull-ups at 6 years old. Seriously....there's a difference between forcing a 1 year old and punishing them for not being potty trained vs. 3 year old and rewarding them.
PS: We put pull-ups on him at night for the first week and we noticed the first night he was dry, and each consecutive night, he was wetter. My night 5, he was soaked....so we took them away, since he'd call them diapers and then correct himself and call them pull-ups. He knew full well that it was still a diaper. He'd only had a handful of accidents after that. By 3.5, he was getting up in the middle of the night to pee if he needed to.