Ser vs Estar in Spanish: Using estar (not ser) when talking about feelings and emotions

In Spanish we use verb estar (not ser) to talk about feelings, emotions, and physical conditions or states of people. 
Have a look and listen to these examples:

Feelings and emotions are considered as temporary states, and this is why we use estar and not ser.  It is possible to have the same sort of sentence with ser and the meaning will change. For example:

In this last sentence Alejandro is not happy temporarily, rather "happy" is his permanent characteristic, i.e he is always smiling and positive.