Motivation (spoilers)

Good theory actually. He clearly has some strong motivation to stalk Clara in particular, even though his past is never disclosed in the movie. While watching, I had the impression he just disliked her for her cheerfulness, but you raise good points. (Maybe they're planning a sequel...)

But I have love in my heart - Yes, as a thief has riches, a usurer money

"This makes me think that he disagrees with the statement and she is not random."

She lives in the same building he works in, so I think that can mean right there that she is not random. Certainly not as random as a random person he might bump into on the street. So, yeah, he did disagree with the statement because he knew Clara and she was not random.

"Only Clara is remaining."

I don't know what that means. But the line comes just after a scene in which he upsets the building owner / manager by killing his flowers. "You should have seen his face." That's the same day he brought grief to the old lady with the dogs by making one of them sick. "Today was a good day, Mum." As I watched the movie I was thinking that he was pleased that he had brought grief to everyone in the building at sometime by that point (two tenants down in one day!), except for Clara.

He also implied something happened with his mother when talking to her about Clara. He said something like this: "She (Clara) will be hard more or less like you (mother) were hard"

My two cents, bye

Good theory, but I took it simply as he wanted people around him to suffer, and Clara simply wouldn't no matter what he did. He was just stating a fact when he said "Psycho, who searches for random victims" because he was 100% targeting Clara at that point. He was just trying everything to make her feel sad. There were a lot of people he didn't bother in that building. He preyed on the happy only, and Clara was the happiest person he ever met. So that made it his goal in life to emotionally destroy her.

also, there's that scene where the old argentinian bastard reviews his work history, one month in one building, 3 months on another, and so on, wich actually makes me believe that indeed Clara was the only one remaining, all other 'projects' were succesful

I think she just happens to be the last one in the block who is still "annoyingly happy". That's it. He's a psycho and doesn't need to show any more motivation. Also she's hot, and that's probably a plus for him.

About his previous life/traumas, it's an aspect which is never revealed, and I think it doesn't take any credibility out of the movie. Quite the opposite, it makes the whole thing even more disturbing, even more real to the voyeur outsider. We only learn throughout the movie about the parts HE wants people to learn (the parts of his story which he reveals in the final letter).

We're never going to know what is really going through his mind, the same as would happen if we saw this story on the news. Disturbingly realistic.

Genious.

He feels he cannot be happy and therefore he wants no one else to be happy either, that's why he always messed with the lives of the people that lived in the building where he worked like when he fed the old lady's dog and got it sick or when he framed the housekeeper son. The thing with Clara was that she was not as easily disturbed as the other people, she was always happy and smiling and he hated her for it that's why he makes his life's mission to ruin hers.