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Probably your browser security settings. I've set mine to 'enhanced' security, and am now constantly re-logging in, on all websites I normally visit.
A bit of advise, somewhat related - Keep your anti-virus up-to-date (both the program & the definition files). There are a lot of SPAMMERS out there, using automated 'bots' which search and surf the web, looking for unprotected systems to infect. A 'firewall' on your system (via the modem/router, and the individual PC - either via OS or AV software) is also a MUST.
I watch my Symantec 'protection logs', and seldom more than a few minutes goes by without an automated 'bot' attempting to infect my home PC.