Why do people change their goals?

when you look at your goal like at a destination you want to reach in your life then this is a very easy question,

when your young you maby have the goal to merry a princess,
have a big house and be boss of your country,
but when you grow older, you see that that goals isnt very easy to get to, the path is just not available, so you ll take another one, you want to study first, then get a good job,buy a house, get a girl, merry, get kids, live long and happy
but when your studying you meet this funny girl, and together you guys see another destination, study,get good job, rent a house, merry and get kids, grow old
but then something bad happens in your live and and you need to rent a house by yourself at the age of 17, then your destination changes again,
every path, every option, every thing that happens on your path towards a destination changes, and so ll your desires, your goals change in life