Things change so quickly and dramatically. Two of my closest friends got married in the last one week. On the one hand, happy for them. On the other, feeling bad that things might change now. They'll move away, get busy with their own married lives and things will surely not be the same anymore.
It's human nature to resist change, isn't it? The subconscious mind is supposed to be a master of this trait - resisting change. Maybe it's just that part of my mind that's playing tricks - feeling bad. The conscious one says "Cherish the good memories and move on. Maybe it's time to make new friends and maybe the void will be filled by that super special one who'll take their place. :)