He is a very good friend, you spent 1/3rd of your day with him for 12 years! Too, he was "parallel" to you and your life, and now he is changing his life and that could be stirring up your own life and what you had wanted for yourself? People we have in our lives for that long, you can't just rip that part of our life out and have no problem with it. You "expect" and are comfortable with certain day-to-day activities and now your day is changing without your having decided to make the change.

Think about the last time you changed jobs or went from one grade to another in school, had girlfriends and then got married, etc. When we are making the change, the people we leave behind don't seem to "matter" as much as when others leave us behind. That's because when we are consciously changing we are looking forward but when others around us change, we are "left" where we are and have only a backward look to remember what it was like.