Why is it that some people just have to disagree with whatever you say?

There are very few foreigners here in Naypyidaw, and about the only one that I know has the "ah but you're wrong" disease.

It doesn't matter what I say - this cvnt has to disagree with me and have the last word.

"I jogged 5km before breakfast, feel really good"
"You will feel bad later - you should never jog before breakfast"

"I just had a nice meal at that Thai restaurant"
"No, their food is crap"

"I bought a new mobile phone last week, 3,000 baht"
"You were ripped off"

"My class of kids are learning really well"
"They are all lazy - you aren't pushing them"

"It's my birthday tomorrow"
"Nope - you only think it is, actually your birthday was last week"

and on and on and on.....

Sometimes I feel like stabbing the guy with a bread knife.

So what mental illness or personality weakness do people like this have, that makes them disagree with whatever comment someone else makes?