It has already had an effect on him, preserving his youthful appearance far beyond what is natural. And he tells Gandalf it has made him feel stretched thin: "like butter that has been scraped over too much bread." Because the Ring has an evil influence, it is concerning to Gandalf that Bilbo is feeling any effect at all.

The ring is evil and Frodo seems like a good guy.

Hearing Gandalf's story, Frodo is frightened and angry, and he wishes aloud that Bilbo had killed Gollum when he had the chance. ... Frodo asks Gandalf to take the Ring, but the wizard refuses vehemently.

“Many that live deserve death. And some that die deserve life. . . . even the very wise cannot see all ends.”

That the ring is worth less and he shouldn't have it.

non above

Because he is more powerful then the ring.

Sam, being a hobbit, and therefore somewhat immune to the Ring, not having any magic, having absolutely no desire to have power, and only having had the ring for a short period of time, was able to give the ring up relatively easily.

Sam created the ring and put a spell on it so it doesn't hurt him.

All above