rather than making something up, I prefer to put it into the context of the song.
The narrator doesn't want to be great or notable ("I don't wanna president, superman or clark kent" -- note that he rejects even the "mild mannered" sort of everyday notability of a newspaper reporter). He's ignorant and unlearned ("I don't know just what I've found, I don't know my sky from ground") and seemingly proud of it, or at least ok with that.
But, he CAN be himself ("how 'bout you?"). ie, he can do the things he's always done. I think in that context, he's just trying to offer all he can. He can be a pleasant moisture, or perhaps, in the end, all he can be is something that will keep her trapped in doors. i think there's two ways to take that. If it's a warm spring rain, maybe he thinks that's a pleasant image. if that's not pleasant, he's either a) the thing that will keep her indoors (ie, with him, because she doesn't wANT to leave) or be the thing that will trap her, but he can't be more. He's just warning her.