Due to the way chiles ripen, the chile will vary from day to day in hotness. The Serrano for example one day will almost be mild and tomorrow will absolutely knock your socks off with heat.
The capsaicin is produced by glands at the junction of the placenta (think of the white veins) and the pod walls. The highest concentration is mostly in the placenta tissue.
There is a misconception that the seeds hold the greatest heat. They do not. You certainly can reduce the degree of heat by removing the seeds because you are in fact removing the "veins" along with the seed.
Burns from the chiles, can be very painful and irritating. Simply because the oil is so volatile, you must use caution when handling chiles.
Chiles can really burn you.
The only real guide to the heat of the pepper is to barely place the tip of your tongue, very lightly, I do mean very lightly "hey it is your tongue" so be careful, on the cut edge of the pepper "hey trust me even the mildest habanero will curl my toes"
When handling chiles you should have a good idea before you start how hot the chili might be, whether it is jalapeño or habanero, a serrano, or a cayenne.
Don´t spoil a great dish by making your guest hurt.
There is a big difference between HOT and HURT.
Yes, it´s true you can get used to the fire of chiles. The more you eat over time the more heat you will be able to tolerate. Be careful if you take a month or two off you are back to being a tenderfoot.
Start with milder chiles and work your way up.