Word for describing the emotion of helping someone

I don't believe such a single word exists.

There are psychological theories that propose that all acts of altruism are ultimately self-interested, because of the feelings of satisfaction that arise. For instance, see wikipedia's article on psychological eogism or altruism. If there were a single word that meant this feeling, it would undoubtedly be invoked in such a context -- but it's not.

Since I've never heard of such a word, and can find no evidence of it existing in fields where it would naturally arise, I conclude that no such word exists in English.

An "empathic throbbing"! That'll do it.

There are many one-word expressions which describes the action of doing something unselfishly for others but as starwed affirmed hardly any in expressing the actual feeling in performing a good deed. However, there are ways round this limitation other than suggested.

Thoughtful Having or showing heed for the well-being or happiness of others and a propensity for anticipating their needs or wishes.

Selfless Having, exhibiting, or motivated by no concern for oneself

Considerate Showing kindly regard for the feelings or circumstances of others

Feelings

Altruism: Pure altruism consists of sacrificing something for someone other than the self (e.g. sacrificing time, energy or possessions) with no expectation of any compensation or benefits, either direct, or indirect (e.g., receiving recognition for the act of giving).

Gratified: experiencing or manifesting pleasure

To feel a warm glow: a warm feeling, as of pleasure or well-being.

A warm fuzzy feeling: informal, full of love and kindness

A glow of happiness: e.g. "Anne felt a glow of happiness as she held her new baby."