Because when someone first leaves, they feel guilty and will make all sorts of promises etc so they don"t feel so bad about what a d!ck they are being. As time goes by they become more comfortable about their decision and develop a sense of entitlement.
It's one of the reasons it's risky to "stand" too long without getting formal financial and custody agreement in place ... walkaways seem to become less generous and accommodating as time goes by.
Red, make sure you do protect yourself financially and in relation to custody arrangements. You can always wind those things back when things improve between u and h - but u need to make sure you are covered in the meantime.
If you haven't already, have a look at the Australian govt child support agency site (www.csa.gov.au) ther are some good resources there to assist in making agreements on custody etc.
