i know that yo is added when you want to show respect for the person.

yes it's more polite only when a Korean sentence is long it's means that it's more polite +yo

end with [~yo],[~(hap)nida], to elder people. to Not friends.

oh . i see . thank you so much :) now i know

사랑해/사랑해요.....you can use both for either case, let it be between the friends or lovers. one is shorter and others little softer ending. both means same. all depends on how you want the sound to be ended. it can be both for same age group or older. it doesn't make much differences.