This is a major difference between western boxing habits, and Muay Thai (at least the Muay Thai of Thailand). In boxing it is very common to protect the body with the elbows and forearms. You aren't protecting against elbows at closer range so your guard can be lower, for one thing, and a crouch can be advantageous in boxing for many reasons, both offensively and defensively. Lots of westerns come to Thailand and favor this habit. But in Muay Thai the body is mostly protected directly by the knees and shins, and the guard stays higher. It's a very different defensive posture.
This is related to some degree also to the hips. One of the biggest challenges I think for a westerner in Thailand is learning how to push the hips forward as part of defensive maneuvers, especially, at closer range. This goes against a lot of western instincts which basically are inclined to pull the groin back (to safety) and to hunch. It can make a bad habit in Muay Thai. One of the concerns of training boxing is getting comfortable with an ass-back defense.
There is a lot of variation in Muay Thai styles, so this isn't universal, but one of the biggest hurdles westerns have in Thailand is the orientation of the hips in both defense and attack. Thais learn early on that pushing the hips forward can be very advantageous and safe. Adding to the western bias towards the hunch is that Greco-Roman wrestling also can favor hip-back, ass-out positions (very different than most Thai clinch positions) so with western boxing and wrestling combined the tendency of the ass-back can get in the way of a lot of Thai Muay Thai techniques, at least at introductory levels. You need to be able to toggle the hips, out and in - you see Saenchai humorously do this in fights, and be prepared to use your shins defensively.