The general meaning of this song is pretty obvious - the speaker is single, unhappily so, and seeing couples together and in love makes her/him feel sad and left out. I think the "you" in "maybe tomorrow YOU'll be mine" could be a hypothetically non-specific "you" - like, whoever it is that I'll someday love and walk hand-in-hand with and kiss in the rain. Or, it could indeed be someone that this person once loved, or someone who s/he still loves but by whom isn't loved in return.
The general meaning of this song is pretty obvious - the speaker is single, unhappily so, and seeing couples together and in love makes her/him feel sad and left out. I think the "you" in "maybe tomorrow YOU'll be mine" could be a hypothetically non-specific "you" - like, whoever it is that I'll someday love and walk hand-in-hand with and kiss in the rain. Or, it could indeed be someone that this person once loved, or someone who s/he still loves but by whom isn't loved in return.