I heard the following: Imagine that you have a friend. You help this friend a lot. You give him a home to live in. You give him food to eat, and clothing to wear. Every need he has, you take care of. One day, you ask him for a favor. How would you react if he said no? You give him everything, and this is how he repays you when you ask for one thing? You would not like it at all!
Hashem gives us everything. Without Him, we’d have nothing. He asks us for a few things in return – 613 mitzvos. How can we refuse? Purely out of gratitude, we should do it!
That is also a pshat in teshuva m’ahavah versus teshuva m’yira.