Demons and angels aren't supposed to get along. Mephisto knows this which is why he insists that this arrangement he has with you is completely professional. He stays out of your realm, you don't bother his. After all, you're an Archangel so you could really screw up his day if you wanted to...

He has to keep the peace as his brothers run wild...That's why he's justifying taking you to this dinner, staring at your wings, nearly kissing you...But yeah, it's completely professional for him...