3 reasons cause us to give:
Our Heart
At the center of our spirit is this reason to give just because we want to. Giving for the following reasons isn't rational:

People we care about
Charities we believe in
Events that tug at you (9-11, Katrina, Sandy, etc...)

The effect of giving from the heart is it makes us feel good.
Tax savings
Who wouldn't want to be able to save on their taxes when they give? As long as we follow the rules of the IRS we can deduct certain types of gifts from our income (most can be found on Federal Form Schedule A). Giving of these resources is rational and more strategic than emotional:

Money to your local church or non-profits like WRLR-FM, home of TechTalk by Michael Kastler)
Items like clothing to GoodWill or other charitable collection organizations
Time volunteering, although the tax savings is minimal and will be found in mileage to a location and possibly other supplies necessary to do the job.

The effect of giving for tax savings makes us feel like we did something smart.
Exchange
We exchange our giving in order to get something or because we have to. This type of giving isn't from the heart and it isn't even smart:

Giving in the name of someone else's benefit - like my wife's company highly encouraging her to donate to the United Way
Fundraisers - for the sole purpose that we can win a prize
Obligation - the feeling we have to reciprocate a gift

The effect of giving because of Exchange is we feel manipulated or are trying to get something out of the action.
The true meaning of giving
While saving on taxes is a good financial choice, the net benefit doesn't make sense.

Exchanging a possession in order to get something else is more like trading that giving.

The best technical definition I have heard about giving is "Freely transferring the possession of something to someone".

Hand it over and let it go. A gift should be made to make someone else's life happier, healthier, and better.
Real "giving" is not rational. It shouldn't be