There's a lot of variables that could be at work here; what browser you're using and how it handles cookies, what the operating system does in handling (Mac and Windows do it a bit differently), how you have you're settings for cookies et al, cache settings, how many sites you visit between XMarks and the security settings. For example, on one machine, I never have to log in. It's using Foxfire with Mac OSX and set to remember specific sites. However, if I go away for a long weekend or a business trip and don't log onto the site with it, I will have to login when I next hit X Marks. With my home desktop running Mac 9.1 with IE, I have caching set to empty at the end of every session so I have to log in every time.

The short answer is that you have some setting that is deleting or not even accepting the cookies for remembering you.