Recalling an example of Harry's keenness, Will said: "One time I remember we went all the way to Birmingham out of nowhere because he decided he needed to see this girl.
"We got a train on the day to Birmingham, got off the train, had no idea where she was going to be, it was raining really hard and we walked across the whole of Leamington Spa trying to find her.
"Then we somehow managed to find an address and it was great, but it was two or three hours of tracking around Birmingham.
"So he goes to great lengths and he's caring, but he's full on. In any relationship he's going to be full on."
Harry has dated a string of high-profile women - including TV presenter Caroline Flack, 33, and most recently 'Trouble' hitmaker Taylor Swift, 22 - and although he has a reputation for being a womaniser, Will insists it is as a result of his pal's fame "situation" and the female attention he receives.
He added to HeatWorld: "He didn't used to be like that - it's probably just because of the situation he's in now. There weren't hundreds of thousands of women throwing themselves at him, that's probably something to do with it."
Harry and Taylor are reported to have split up at the beginning of the year after a massive row during their New Year's vacation in the Caribbean and Will believes the pressure of their fame is what caused their break up.
He explained: "When you've got two of the most famous people in the world, I don't think he knew what that was going to be like. It's doubled his chances of everyone wanting to see him.
"Everyone wants to see him and everyone wants to see her, so when they're both together it's going to be insane."