Zacchaeus climbed a tree because he was too short to see Jesus in a crowd. Jesus found him and told him that he was going to stay at Zacchaeus' home. Zacchaeus was so happy, but others were surprised because Jesus said that he would stay at a sinner's home.
Zacchaeus needed Jesus so much, it must have been embarrassing to climb on a tree but he did not care, and he changed a lot when he saw Jesus. It seems this is a case that a camel went through a needle's eye.