Why? The need to validate oneself on the internet. The desire to put one's own opinion online, without having to pay much regard to the thread starter or any other posters replying to the thread.
You read a thread title and go 'Oh! I have an opinion on that! Me! Me! Me! And SOMEONE, but I don't care who, needs to read MY opinion! So I must put it out there!'
That's how it goes on subconsciously. There's a lot more psychology to it, and it differs from individual to individual, but that's what it boils down to 99% of the time.
It's not necessarily always a BAD thing per se, in the sense that it's usually not motivated by any malice toward the thread starter or the rest of the community, but it is kind of insensitive. I know I've been guilty of it in the past, though I usually read the OP and every single post in the thread before replying.
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clarity of thought before rashness of action