why does iago say that he loves desdemona?

I think Iago simply means he "respects" her and likes her. The word "love" was used in different contexts in Shakespeare's day.

I don't think Iago ever says he loves Desdemona. At the end of Act II, scene i, he explains that he lusts after her, but also says it's because he wants to get even with Othello “wife for wife” (II.i.286). He believes Othello has been with his wife (Emilia), and he wants revenge.