Why is there a से at the end of तुमसे? I thought that would mean "I will always love from you"
you are not wrong in that "से = from you" but in the right context as in the above sentence it means "to you or you".
शुक्रिया. Is there a specific reason why से is here, or is it just how प्यार करना is used?
yes, that is how you say love. There are a couple more ways. You can say" Main tumKO hamesha pyar karoonga" or "Main tumHEIN hamesha pyar karoonga". All three mean the same in the end.
तुम्हें is another way of saying तुमको, correct? Both of them are interchangeable?