"Hey, long time no see. How've you been? What's that? You came all the way here to celebrate my birthday? Thanks! I'm really happy!" He goes on to point out that of all the animals, only humans have this tendancy to celebrate birthdays and other special days. He rhetorically asks what you all get out of celebrating someone else: "Is it pure pleasure and fun? A chance to relate everyday thanks? Is that really all it is?" In the end, since you insist on celebrating, he has no objections—after all, as you may know, he loves the human race.
Happy birthday, Izaya!