Lots headcanon Izzy and Ed growing up as children/teens together. But consider for a moment that their actors have a ten year age gap, and so what if the characters did as well.

Imagine a 26 y/o seasoned pirate Izzy showing 16 y/o fresh-off-the-heels-of-killing-his-father new pirate Ed the ropes. Izzy takes on a mentor role towards Ed, and for Ed this is the first positive male figure he's had so far in his life.

And then years pass, Ed is no longer a teenager, he's almost/past 30, he's not working under Izzy's wing anymore, he's about to pass him in rank.

And something clicks in Izzy's brain that Edward is an immensely attractive man.

This thought is met with immediate internal conflict because Izzy cannot mentally separate Ed-the-adult with Ed-the-teenager, so Izzy feels ashamed of himself as though he had been lusting after 16 y/o Edward who he's responsible for and not 29ish y/o Edward who's responsible for himself and other members of the crew. It was fully-adult-and-in-charge Edward that caught Izzy's eye, but being able to remember when Ed wasn't that makes Izzy doubt his own intentions.

So Izzy represses his feeling for years, never acting on his feelings towards Ed except through acts of assistance, reinforcing his former mentor role. Izzy acts with an acknowledgement that Ed is competent and capable, but also with the reminder that Izzy is there in case Ed needs any help, which is big mom energy (but for some reason I'm thinking of the mom Amy Poehler played in Mean Girls?)

And maybe at some point Ed actually did try to initiate a relationship (or tryst) with Izzy, but Izzy literally wouldn't let himself accept it. Ed's well into adulthood at this point, but Izzy just won't stop thinking of him as the teenager in crisis he met all those years ago.

And this does explain a why Izzy was at times treating Ed like a petulant child, because Izzy has been scolding Edwards since he was one.