Here are the reasons I think it makes them feel good:There's nothing in the episode that specifically says that Fellicium makes you feel good.
- The Ornaran's can't repair their ships. If it doesn't make them high at least it impairs them.
- Dr. Crusher said they're "feeling fine, in fact too fine".
- Dr. Crusher describes it as a narcotic.
- They look happy when they get their injections.