Is it just a personality thing? Or just a normal horse thing (they're herd animals, etc)? More importantly, can I change it?
As an owner of horses for 57 years, I am going to waste my time and answer this ^^^^
No. You cannot change whatever the basic/inherent thought process of each individual horse is.
Some are more people friendly than others BUT------ one very important thing separates them from domesticated dogs:
Dogs are basically predatory meat-eating creatures.
Horses were, are, and always will be prey animals, no matter how much bubble wrap we try to put them in.
They will still kick, if they feel the need and aren't properly "horse disciplined.
They will still bite (BTW, you DO know their jaws lock when they bite onto something), if they aren't properly "horse disciplined", and they will still run away in fear or lack of proper "horse discipline".
I treat my horses with utmost respect and fairness. The medical care they receive is better than a lot of people's children get.
The horse in my avatar was my heart horse for 24 of his 27 years. He is laid to rest on his own property.
BUT they are still livestock ---- unpredictable livestock. I would no more think about trying to make them "dog like" than I would think about making a big dog "horse like".
About the time anyone thinks they can re-wire the thought process of any species to resemble another species, it is asking for trouble of some sort. I can easily envision an over night trip to the local hospital where horses are concerned.