these young people become part of one's daily community, there's no doubt about that. You come to love them, you see them through a lens similar to the one you see your own child through. If you didn't feel love for this young man, and pain at his absence, I'd venture to say THAT is the sign that there's something amiss with you emotionally!
I think it's a real commentary on the English/British culture that we think to sneer at people for feeling pain when a member of their community (someone who is at the house constantly, who you've got to know, who you've bought gifts for, cooked for, etc.) suddenly just isn't there anymore!
"oddly cosy" JFC, one's home should be cosy for a friend who is in and out of your house constantly for two years SHOULD be missed if they suddenly aren't there anymore!
Do we not allow ourselves to feel love and attachment in case we become too "oddly cosy" with a young person?! And we wonder why there is an epidemic of loneliness in this country...
OP as long as you're not revealing the extent of your pain to DD, you are doing fine! Feelings are not illegal, and pain is the price we pay for loving. Feel your feelings, mourn the absence, and take heart, you (and DD) won't miss him forever and life will go on. Focus on DD and don't let these strong feelings spook you. They'll pass.