How did Jonathan love David?

Without more background, this is a difficult question to answer. Who is Jonathan? Who is David? Are they married? Is this from a book? From real life? Is it meant to be only answered by one person, or meant to be answered in a specific way? However, I can say that this would vary based on what time period you’re talking about.

For example, if this is a father-son relationship in, say, a rural community in the 1960s in America, the father - Jonathon - probably sent David to a good school. Probably went to all the school activities he was participating in.
However, if it was a man to man relationship in the same time period, they couldn’t get married based solely on the fact that they are two men. Again, context is everything.