Why does my adaptor get hot

It's normal for any device that converts AC power to DC power (or even DC power to DC power, like a car charger) to generate heat as part of the conversion process.

If you are concerned it is too warm, you can make an appointment to have it examined:

Genius Bar Reservation and Apple Support Options - Apple

It’s called the Second Law of Thermodynamics. Anything that creates or uses energy does so with some loss. That lost energy is expressed as heat. So when you charge the phone it generates heat in the power source, heat in the charger circuit in the phone, and heat as the energy goes into the battery. The faster the charging, the more heat is generated. Likewise when you discharge the battery; not all of the energy from the battery gets to the circuits that use it. Some of it becomes heat. 

I’m assuming you are using an Apple adapter or one certified as Made for iPhone; likewise for the charging cable. If either is not Apple or Apple certified don’t use it and destroy it. Why Counterfeit Lightning Cables Kill iPhones - Motherboard