As mentioned here, Gear Second decreases the user's life span. When he first used Gear Second, it was a really important decision to make since it's side effect is so brutal. But after Enies Lobby Arcs he started using it casually. What changed and made Gear Second from a very risky technique to a casual technique?
The main cause is training, and mastering of the technique.
After the timeskip:
After training with Rayleigh, Luffy is capable of using Gear Second with specific body parts, activating the technique without using the stance we see in Enies Lobby Arc. This way reducing the strain he imposes on his body, making him able to use it withouth the side-effects, it is even stated in the manga that he could use it on his entire body with only the hungry side-effect.
This only explains why he is able to use Gear Second so many times. The real reason for using this technique so much is because of the formidable opponents he faces. He needs to be able to match or overcome them.