Imagine that you had to travel and the ticket cost $2500 and you had no money to pay and yet you must take the trip. And out of the blues, someone who knows the importance of the trip to you went out of the way to inconvenient himself to get the ticket for you so you can travel. Would you say that because He paid the fare that you wont take the trip? Or would you want to repay the fare for the journey?
The Lord's death on the Cross was to purchase the ticket for our destination 'Eternal Life' wouldnt you rather enjoy the ride? Why want to pay for something you cannot pay for?