If you're happy with XP and don't feel you want a more modern user interface or play the latest video games then, really, there is no reason for you to upgrade. Eventually (2014) support for XP will be dropped and no further security updates will be created for it. At this point, it would be wise to upgrade if you hadn't already.
Windows 7 is better than Vista, as a product. However, whether it's "better" than XP is down to user preference. XP is uncommonly refined for a Microsoft OS and this has been a problem for Microsoft, as a replacement operating system is a hard sell if the last one was particularly good.
Bottom line, keep your XP for now, it's tried and true and you're not the only one who thinks so.