Some modern critics suggest Jonathan had a homosexual fascination with David; others say the two were lovers. Their evidence is the verses in which the two declare their love for each other, and in which Jonathan clearly seems to idolize David even to the point of betraying his own father for David's sake. This is the kind of forced interpretation that, in other contexts, gives midrash a bad name. Anyway, I think its more likely Jonathan had an ordinary heterosexual man crush on David.
What's that? If you're a man with ordinary powers of observation you've likely seen such crushes develop at work or shul. One guy, usually a beta, starts shadowing an alpha. The beta contrives to always walk home with the alpha, invites himself over to the alpha's house, and may even begin echoing the alpha's speech patterns. If the beta is a loser, who comes on too strong, the man crush dance looks pathetic and makes the alpha uncomfortable -- but not every man crush follows that pattern. Sometimes the beta plays it cool and the alpha enjoys the attention. Sometimes the beta has something to offer the alpha. Sometimes he and the alpha become good friends. Such things happen whenever and wherever males interact. The apes do it too.
The David and Jonathan bro-mance seems to have begun with such a man crush. David is obviously an ultra alpha. He's charismatic, people flock to him, and no other man in the bible is said, in so many words, to be loved by a woman. In fact, as the verses make clear again and again, everyone loves David, especially Jonathan, who though a prince, and a winner on the battlefield, doesn't seem to posses powers equal to David's. Instead, he falls under the usurper's sway and starts to idolize him. Being in David's company makes Jonathan feel whole. He wants to be around David, and he wants to do things for David. This is a a pretty typical man crush behavior with mo homosexual aspect required.