Do you get Spam from people, who do a regular login?

Why do I have to solve a captcha, after going through a login?

Why do I sometimes get an captcha and sometimes not? Can't the system see, that there has never been a spam from my account, but that the foolish robotdetection is constantly suspecting me wrongly being a robot?

It's so annoying!

How does this detection work, and what can I do to avoid captchas?