In this post I list a number of motivations for giving that the Buddha discussed and ranked according to their kammic potential. I explain in brief why Theravada Buddhists believe that skilful giving benefits the giver more than the receiver, and why the Sangha of the Buddha’s Noble Disciples are considered the most worthy recipients.
In the Dana Sutta, it is said that the Buddha discussed a number of motivations one might have for being generous and that he ranked them according to their kammic results. The motivations in order of ascendency are:
Giving with the thought, “I’ll enjoy this after death.”
Giving with the thought, “Giving is good.”
Giving with the thought, “This was given in the past… It would not be right for me to let this old family custom be discontinued.”
Giving with the thought, “It would not be right for me, being well-off, not to give a gift to those who are not well-off.”
Giving with the thought, “Just as there were the great sacrifices of the sages of the past… in the same way will this be my distribution of gifts.”
Giving with the thought, “When this gift of mine is given, it makes the mind serene. Gratification and joy arise.”
Giving with the thought, “This is an ornament for the mind, a support for the mind”. 
Among these motivations for giving that the Buddha discusses, it is the last one he regards as being highest.  Why? My understanding is, when there is full realisation that one’s gift is “an ornament for the mind, a support for the mind” such a gift is selfless, completely free of attachment, and while such action (kamma) may bear wholesome fruit it doesn’t result in the giver being reborn in this world. Although the gift has some utility value, the giver is able to make the sacrifice without heartache or reluctance because s/he knows it ultimately as anicca, dukkha, anatta.
However, when discussing the other motivations the Buddha’s conclusion is they will each result in the giver being reborn in one form or another when the wholesome fruits of her/his generosity are eventually exhausted. So from the Buddha’s point of view they are less virtuous motivations, though still worthy. 
According to my understanding, Theravada Buddhism maintains that the most important thing about any action is its effect upon the doer’s state of mind; the effect it has upon others is a lesser consideration. It is for every person to take responsibility for their own state of mind. Why? Because ultimately each and every one of us is solely responsible for how we think and react to what other people do or don’t do.
One cannot therefore relieve another’s suffering by giving them a gift, for example, because for anyone to receive something “pleasant” or “unpleasant” is vipaka – the result of their previous meritorious or non-meritorious conduct.  To give skilfully, however, is wholesome kamma and it produces wholesome vipaka; one enjoys the wholesome result or fruit of one’s own generosity for as long as it lasts – either now or in the future.  The Dana Sutta quotes the Buddha himself as saying, “This, Sariputta, is the cause, this is the reason, why a person gives a gift of a certain sort and it does not bear great fruit or great benefit, whereas another person gives a gift of the same sort and it bears great fruit and great benefit.” 
Writing on the practice of giving, Susan Elbaum Jootla states: “The most excellent motive for giving is the intention that it strengthens ones efforts to attain Nibbana. Liberation is achieved by eliminating all the mental defilements (kilesa), which are rooted in the delusion of a controlling and lasting ‘I’. Once this illusion is eradicated, selfish thoughts can no longer arise.”
And as she also goes on to explain, gifts may be material or spiritual and big or small.  (The Buddha regarded the gift of Dhamma as excelling all others). The ariyas or noble ones are the worthiest recipients of gifts because they remain detached from the objects presented. They are compassionate however and so they accept gifts in order to provide opportunities for donors to earn merit. Moreover, their wisdom and purity of mind makes the act of giving capable of yielding “abundant benefits.”  (Supporting the Sangha of the Blessed One’s disciples will enable others in this world to hear his supreme gift of Dhamma, for example.)
To conclude, the highest motivation for giving in Theravada Buddhism is founded upon the ultimate realisation that any attachment to one’s gift is a fetter that keeps one bound to samsara.
The most important thing about giving is its effect upon the doer’s state of mind; the effect upon the recipient is a lesser consideration.
Monks who are aiming for Buddhahood strive to eliminate all attachments through the “perfection of giving” (danaparami) – by giving away the dearest of possessions whenever the opportunity arises. Such giving is true renunciation; their generosity is motivated by a sincere desire to end the suffering of repeated existence and it completely disregards the qualities of the recipient or the potential fruits arising from meritorious conduct.
The Sangha of the Blessed One’s disciples are the most worthy recipients of gifts, therefore, and much merit-making can result from skilful giving that supports the Sangha’s wise and compassionate efforts to teach the Buddha-Dhamma and preserve it for future generations to hear.
 ‘Dana Sutta: Giving’, translated by Thanissaro Bhikkhu
 ‘Giving from the Heart’ by M. O’C. Walshe
 ‘Dana Sutta’, ibid.
 ‘The Practice of Giving’ by Susan Elbaum Jootla