Here's why:
* It's not a proper noun: "Sweetheart" is a descriptive term, not a name.
* It's not a title: Unlike "Mr.", "Mrs.", or "Dr.", it doesn't signify a specific position or role.
* It's informal: "Sweetheart" is a casual, affectionate term, and capitalization is usually reserved for more formal language.
However, there are some exceptions:
* Emphasis: If you want to emphasize the word "sweetheart" for a specific effect (like in a dramatic scene), you might choose to capitalize it.
* Personal preference: Some people might choose to capitalize it out of habit or because they prefer a more affectionate tone.
In general, it's safe to assume that "sweetheart" should not be capitalized when addressing someone.