Why does my pan always splatter when I make chicken breasts?

The reason why fried meat splatters is that correct frying temperatures (~ 180 Celsius pan surface) are well above the vaporizing temperature of water (100 Celsius). Each droplet of cell plasma which comes into contact with the frying fat creates a tiny steam explosion. And explosions splatter.

If you were meaning to ask not why it splatters, but how to prevent it: you can't. See also the existing question about it.

On a side note, 12-14 minutes per side of chicken breast sounds like you are reducing it to particleboard. See this question for correct internal temepratures.

In addition to @Rumtscho's answer: chicken breast is often injected with water to increase profit, especially when you buy it in discount stores.
When you heat it, that water turns to steam, and if it leaks first into the hot oil or butter you get serious splatter.
It's bad enough some people call it "exploding chicken", and radical vegan organisations use it as an instrument to try to force laws against eating chicken, and especially against selling meat at a discount.

Frying time would of course depend heavily on the thickness of the chicken breasts as well as the heat you're using. And whether you spread them open before frying, if you don't they're twice as thick and will take a lot longer (and can easily be blackened on the outside while still raw on the inside).

Personally, I've had good results putting chicken breasts in the oven. Spice as desired, then fry lightly to get a nice crisp crust on them, cook in the oven at 175C for about half an hour to get them well done and still nicely moist.
No splatter that way either.

A little trick my mother taught me - add a pinch of all purpose white flour to the oil before adding in whatever you're frying. It won't totally eliminate the amount of splatter, it will greatly reduce it, although I'm not sure of the science behind it.

Beware though, that adding the flour will tend to burn the flour, so do be mindful of temperatures. About 160C (320F) for cooking oil temp with flour. I typically shallow fry on the stovetop for about 10-12 minutes total for chicken breasts and then transfer them to a 175C (347F) oven to finish cooking.