My boss won t transfer me to my new site and I don t know why.?

So I work security for a great company I do a lot of extra work I work very hard and take my job serious. My boss seems to be on good terms with me, but of late I am feeling a bit bummed because I was offred another position in the company one post I really wanted. And he dosent want me leaving the post I am currently at. I don t understand why and that hurts a bit because I work extra hard and he knows I hate this post.. I been a good and loyal employee and I don t understand why I am not the lead at this site and I am someone whom can be replaced there...

I feel as if maybe they want me to leave but am probably wrong but just wanted a opinion. Why would a boss do that to a good employee