Changes.

Time flies. The changes in our lives are main reason we notice that (beside the wrinkles).

There is a moment when everything once stable and sure starts to change, and we lose the safe structure of your childhood. 
Someone passes away, someone else moves abroad, and all your kid's certainties are challenged from the foundation. 

 This is not necessarily a sad process, but surely it is a melancholic one. 

It is as well true that the only way a human being has to proceed, develop and progress is through changes. From this point of view they are our best friends: they bring us toward the person that we have the potentiality to be. 
To live below our potentialities is a source of frustration. Often we do not even recognize it, dragged as we are by the force of the habit, one of the strongest influence on men and women lives. We settle, we stop to look for something and we try to convince ourselves that we actually found something, even if the evidences point to the contrary. This process goes on until a change comes to destabilize us. 

Therefore, welcome to changes.

 They could not be what we wanted, but they are almost always what we need.