How to make my pitbull like weight pulling more you like to ask?

I have a 12 month old pitbull and ive started him weight pulling a 5lb weight on a sled. i will be using light weights till hes 18 months. can some people tell me what their dogs wetre like when they first started. i incorage him heaps saying work boy and good boy as much as i can but when he pulls he doesnt look like he likes it much even with me trying to excite and praise him. will he learn to like it more. can some people tell me what there dogs were like at the start and what they are like now thanx heaps!!