(For that matter, I've come across many people over the years who have had a sincere interest in Judaism. They wanted to understand more and observe more. Yet they often would see religion as a meaningful diversion -- almost as a hobby. If a lecture or Jewish event occurs in the evening or on a Sunday, they would be more than happy to attend. If, however, it interferes with work -- nothing to talk about. (I knew one such person who had a real but casual interest in Judaism, yet because of the primacy of his career, found himself entertaining an important academic associate for the entire day of Yom Kippur.) I hate to put it this way, but one has to really question if such people are really ready to make a serious commitment to Judaism. Judaism as a *way* of life is truth and meaning; as a diversion from life is glorified bird watching.)
Judaism asks what for many of us is the ultimate sacrifice -- our hard-earned incomes. Not only must we hand it over to G-d, but we must give of it freely to people who expended no effort over it -- to the poor (and of course it's their *own* fault they're poor, etc. -- we seem so qualified to play G-d when people come to our door), and to the non-working tribe of Levi.
The message is thus, clearly, that we are no more than custodians of our wealth. It is all a gift from G-d. He could have just as easily bequeathed it to the poor man as the rich. He granted it to us in order that we use it in the manner He intended -- that we help those less fortunate and those who -- as the Levites -- are devoted fully and entirely to the study of Torah.
Lastly, if we fail to understand this message -- if we fail to recognize the true source of our wealth -- G-d will find need to remind us. Thus, if some tithe while others do not, a partial famine will ensue. (G-d will take down the stock market, cool off the housing market, etc. -- He has no shortage of means.) Only those who withheld from others will suffer. If no one tithes, a more widespread famine will ensue, resulting from some other type of catastrophe, such as war or revolution. Finally, if even the tithing of dough is neglected, total famine will follow, resulting in widespread suffering and loss of life.
But G-d does not only punish us to remind us of this message. He rewards as well. The Talmud writes that charity is the one commandment that we have the right to "test" G-d (Ta'anis 9a). In the Book of Malachi G-d exhorts the people: "'Bring all the tithes to the storehouse (of the Temple)... and test Me in this' says the L-rd of hosts, 'if I will open for you the windows of the heavens and pour out to you blessing without limit'" (Malachi 3:10).
Ordinarily, we have no right to test G-d. ("I will believe in You only if You give me a sign," or "I will keep the Torah only if You give me what I want" etc.) G-d does not give free handouts. We pray to G-d and He is merciful, but this world provides no assurance that He will answer -- or that the answer will be yes.
Charity, however, is different. We have the right to test G-d: to give more charity and fully expect to see results. (I leave this as an exercise for the reader.) ;-) It is thus literally in our hands to make G-d more evident in our lives -- to give to Him and visibly see Him give back to us. Charity thus provides us with the opportunity -- the privilege -- of letting go: of giving over a part of ourselves to G-d -- and in the process of allowing G-d to enter our lives.