Zacchaeus wasn't exactly very popular. Sure: he was wealthy... But apart from those who wanted to leech off him, he was almost universally hated. As a tax collector (a "chief" tax collector at that!), he represented oppression (the ruling Romans occupied the land), and exploitation. He was considered a traitor and a thief (many tax collectors padded their pockets by charging more than the going rate). Tax collectors were considered the lowest of the low (as in this parable ). 

Having heard that Jesus was coming to town, Zacchaeus was curious to meet him -- or at least see him in action. He likely had heard the rumors: that this was no ordinary man. But how could he get the chance to see Jesus while avoiding the abuse that often resulted from being in a crowd? He "ran ahead" and climbed a tree! Considering that the culture of the day frowned upon grown men doing either, Zacchaeus likely climbed the (thick-foliage) sycamore-fig tree with the intention of hiding.

Just imagine his surprise, then, to be called down from the tree by none other that Jesus himself! But he wasn't the only one surprised. We see both great surprise and the common (dis)regard for tax collectors in the response of the crowd - who don't seem at all pleased that Jesus should have any association with Zacchaeus.
Luke 19:1 (ESV) Jesus entered Jericho and was passing through. 2 A man was there by the name of Zacchaeus; he was a chief tax collector and was wealthy. 3 He wanted to see who Jesus was, but because he was short he could not see over the crowd. 4 So he ran ahead and climbed a sycamore-fig tree to see him, since Jesus was coming that way.
5 When Jesus reached the spot, he looked up and said to him, “Zacchaeus, come down immediately. I must stay at your house today.” 6 So he came down at once and welcomed him gladly.
7 All the people saw this and began to mutter, “He has gone to be the guest of a sinner.”
8 But Zacchaeus stood up and said to the Lord, “Look, Lord! Here and now I give half of my possessions to the poor, and if I have cheated anybody out of anything,I will pay back four times the amount.”
9 Jesus said to him, “Today salvation has come to this house, because this man, too, is a son of Abraham. 10 For the Son of Man came to seek and to save the lost.”
Everyone (including Zacchaeus) thought that Zacchaeus would have to "become good" before Jesus would want to talk to him. Jesus surprised everyone by wanting to associate with Zacchaeus right away, and letting Zacchaeus become good when he was ready.

...and that seems to be exactly what happens with people who have spent time with Jesus.