You fall in love and decide you want to be together - but the decision isn't just about you and him. One - maybe both - of you have children from a previous relationship. In the early days of getting to know each other, it's easy to ignore the impact this will have on all your lives.
But as soon as you know it's serious, your relationship seems to recede into the background. You start to grapple with other issues: you may want to spend your time getting to know all about him; instead you have to start by getting to know his children.
There'll probably be an ex-partner still on the scene, who may be downright hostile to your good intentions. The children themselves may be confused, angry or frightened, unwilling or unable to share you with their parent.