Why can't love and affection last forever? I love the dating part - when you just want to hold and kiss them all the time, but I'm afraid of marriage because I see so many broken marriages where people get bored and don't have the same affection anymore. Is this normal?

It's called the honeymoon effect and it's normal. It's a fantastic time in a relationship, but it does fade. People mistakenly make commitments during this period and then realise that the honeymoon effect influenced them. When you are ready to settle down, you are ready to appreciate the "depth" and stability of your relationship.