新手求教 关于express session跳页面后丢失的问题

我在登录的时候用户信息存进session req.session.user=itemDetails[0];
然后 console.log(req.session); 信息如下
{ cookie:
{ path: '/',
_expires: Sun Feb 09 2014 17:00:47 GMT+0800 (CST),
originalMaxAge: 2592000000,
httpOnly: true },
user:
{ ID: '123',
LATMELE: null,
NAME: null,
ADDRESS: null,
PS: '123',
LOGINNAME: '123',
PHONE: null,
LATMWA: null } }
然后在页面点击链接后在后台拿不到session的值，console.log(req.session); 信息如下
{ cookie:
{ path: '/',
_expires: Sun Feb 09 2014 17:00:48 GMT+0800 (CST),
originalMaxAge: 2592000000,
httpOnly: true } }

请问只是为什么