Getting emotionally involved with people is what happiness in life is all about!
But then it can be the biggest cause of misery, assuming you have basic physical needs met.
We need to try and protect our emotions, by not getting too attached to people until we are sure it is reciprocated, and we can trust them.
But even then you can always be misled. Think of people who put their self on the line in a relationship and then it goes sour. Things always change.
Basically a person needs to maintain a degree of self-definition, i.e. interests, self-value, etc, independent or irrespective of other people or circumstances.
Brutha is right on in that another v.important tool is the ability to feel emotions and not let it matter so much. This is one powerful benefit of meditation.
In my opinion a great degree of mental sanity involves being able to act regardless of how we feel emotionally. Easier said than done. Imagine last time you felt angry!
And reason and emotions cannot easily be separated. Rationality might be "feeling appropriately," as in "the right emotion, to the right degree, towards the right thing, at the right time" etc.