In the matter of salutations and signatures in correspondence, many people are confused about comma placement. Here is how the standard rules go, and why.
In Dear Kitty, you are addressing a person (the technical term for this is vocative) and are declaring her to be dear; it is an adjective, and you don’t put a comma between an andjective and what it modifies. Saying Dear Kitty is like saying Sweet kitty as in Sweet kitty, won’t you come lie on my lap?
In Hi, Kitty, the Kitty is again in the vocative, but Hi does not modify it; Hi is an expression of saluation, a performative. Salutations are self-contained in much the same way as imperatives, and the vocative is effectively an interjection; if you want Kitty to listen, you say “Listen, Kitty,” rather than “Listen Kitty,” and likewise it’s Hi, Kitty, how’s your cat rather than Hi Kitty, how’s your cat (unless her name is Hi Kitty). It’s true that many people leave the comma out there; that’s not considered standard, however, as there is a structural disjunction.
In a closing signature, the name is yours, so you are not addressing anyone with it; the signature function is a particular performative, sort of like Amen. It closes the text and expresses that it is from you. (We don’t do it in direct personal speech because it would be silly – it’s obvious that you’re saying what you’re saying.) The Love is short for “with love,” which means “I am sending this to you with love,” so it’s also a performative – but a different one. If you leave out the comma, you are making a direct connection between Love and Kitty, making it read like an imperative: Love Kitty! (With Sincerely it would be less snicker-worthy but still mistaken to leave off the comma: Sincerely Kitty would mean “I sincerely am Kitty” rather than, as you want, “I say this sincerely, and sign it Kitty.”)