Why is grendel excited to see his men arrive

He feels very excited because the arrival of new men gives him the chance to fight with someone new. He has someone he can test himself against.

He has been living in isolation for a really long time, that he feels that excitement even if the men scare him.

Grendel is excited when the men arrive because it is a fulfillment of a premonition he had. He's excited because he believes the men have to come to free him from his boredom.