This is a good question, especially in view of the fact that Eumaios is a faithful friend. 

If you recall, Odysseus was away from home for a long time. Not only did he fight in the Trojan War, but he also had a hard time getting home! For this reason when he returned, he did not know who was faithful to him. Times change as do people. So, he wanted to disguise himself. 

In addition, Odysseus comes back in a much weaker position. For this reason the element of surprise was something that he needed to work to his advantage.