The tragedy of Giovanni's Room is inseparable from the fact that it is a love story. The characters in the novel cannot choose whom they love (it just happens), and the result is that they often experience love as a burden. They feel helpless before their love, and thus the people that they love also make them feel helpless. Romantic love gets stirred up with indifference and hatred and jealousy. Let's just say it's not your typical love story. No one lives happily ever after, nor is it clear that all characters want to do so.
Love is a burden for David because it is inseparable from sexual desire. David cannot choose who he is sexually attracted to, so he cannot choose who he loves.
David brings his desire for distance from his father to his relationship with Giovanni. He imagines love as being inseparable from a degree of "merciful distance," and it is Giovanni's failure to recognize this distance that forces David to leave.