Speaking to an Irish radio station, Harrington noted the frostiness that began over a rules incident with Garcia's countryman Jose Maria Olazabal in 2003 and heightened with Harrington's wins over Garcia at the 2007 Open and the 2008 PGA Championship.
"I gave him every out I possibly could have at the 2007 Open," Harrington said to 2FM's Game On radio show [as transcribed by IrishGolfDesk.com]. "I was as polite as I could and as generous as I could be, but he was a very sore loser. And he continued to be a very sore loser.
"So clearly, after that, we have had a very sticky wicket after that. The Ryder Cup [as teammates] improved it after that but we say hello to each other every day and it's through gritted teeth, there is no doubt about it. I know he is watching what I am doing and I am watching what he is doing. It is one of those things. He's a rival."
Harrington overcame a 6-shot deficit to Garcia at the 2007 Open at Carnoustie, then defeated him in a four-hole playoff. He also won the PGA Championship the following year at Oakland Hills, playing with Garcia on the final day and prevailing by 2 strokes.
Asked to explain their strained relationship, Harrington, 45, said: "It's very simple. Myself and Sergio have been on tour as long as each other. We would have been the opposite. His is a very flamboyant game, everything comes easy. There were periods he never practiced. We were such opposites. I worked at it, grinded it out. Got the best out of it.
"I'm very strong on the etiquette of the game, so I don't tolerate people spitting in the hole, throwing their shoes or throwing golf clubs. That would be my attitude. And it would be quite clear where I came from."
Harrington recently underwent shoulder surgery and hopes to return at the end of May and to be ready for The Open at Royal Birkdale, where he won his second major title in 2008.
He was working at the Masters for Sky Sports.
"I was delighted to see the emotion on the 18th green," Harrington said of Garcia after defeating Justin Rose in a playoff. "Maybe I am a bit harsh with the fact that I look and say, well, everything comes easy to Sergio. But clearly it hasn't come easy to Sergio. You could see in that moment in time that he has paid his dues. I might have had a chip on my shoulder about that.
"Had Sergio paid his dues? I suppose he was a bigger star than he was performer. He has definitely paid his dues now. I could see it in his emotion. His future wife [Angela Akins] -- I met her at the Ryder Cup -- she is a lovely girl. I feel for them. Everyone likes the parents. The genuine emotion and the thrill of winning, I could see that as a competitor and appreciate that. I was very happy for him, no doubt about it, in that moment."