Q. 'Can that person be in trouble for doing it?':
A: Definitely. It's a criminal offence, under the Computer Misuse Act 1990, to gain unauthorised access to electronic data.
Quick summary here:
http://www.theteacher99.btinternet.co.uk/thete acher/gcse/newgcse/module8/task9.htm
Full legislation here:
http://www.legislation.gov.uk/acts/acts1990/Uk pga_19900018_en_1.htm
Q: 'Is it my fault?'
A: Possibly. If you decide to leave your life savings, in a clear plastic bag, hanging on your gatepost, the person who steals it will be the only one committing a criminal offence but you'd probably find it hard to disagree with someone who suggested that you'd only got yourself to blame ;-)
Choosing an 'obvious' password is just as silly as leaving your money where someone can steal it.
Chris