Why does Grendel greet Beowulf as dear long-lost brother kinsman-thane?

Grendel greets Beowulf as a dear long-lost brother kinsman-thane because he is trying to deceive Beowulf and gain his trust. Grendel's intention is to lure Beowulf closer so he can attack him when he least expects it. It is a tactic to catch Beowulf off guard and take advantage of the situation.