Questions on Self

When we are unhappy with another person, why do we punish ourselves by doing things like gorging on food and maintaining anger? 

When we can accept that there is a plurality of viewpoints and opinions in the world, why can we not accept the fact that your loved ones, as a result of this plurality, will not share the same viewpoint as yourself? 

Again, when we are intellectually aware of this plurality and other similar facts (or cliches, if you will), why can we not put words into practice?