Here are some of the reasons why:
* He doesn't fully understand its power: Bilbo initially discovers the ring's ability to make him invisible, but he doesn't realize its true nature as the One Ring. He doesn't understand its connection to Sauron or the dangers it poses.
* He enjoys its benefits: The ring's ability to make him invisible allows him to get away with things he wouldn't otherwise be able to, and he finds this quite enjoyable. This highlights his initially selfish nature.
* He fears their reaction: He suspects the dwarves might not understand the ring's power and might even take it from him. They are already on a dangerous quest, and adding a powerful artifact to the mix could make things even more complicated.
* He doesn't want to be burdened by it: Bilbo feels a growing sense of unease about the ring, and he doesn't want to be responsible for its potential dangers. Keeping it a secret allows him to avoid that responsibility for a while.
It's important to note that Bilbo's secrecy ultimately becomes a major source of conflict and tension throughout the story. The ring's power gradually corrupts him, and his attempts to hide it from the dwarves lead to dangerous situations.