The pasuk says, “v’yikchu li terumah, me’eis kol ish asher yid’venu li’bo, tik’chu es terumasi.” Rav Moshe Feinstein says that in general with regard to mitzvos, a person can take the approach that, although he/she would love to eat a prohibited food or do a prohibited act, the torah commands us otherwise. However, with respect to tzedakah that is not the case. The pasuk tells us v’yikchu li. We must condition ourselves in such a manner that even if tzedaka funds were to be taken from us without offering them up, it would not impact our willingness to give them. We are commanded to become givers by nature so that even a v’yikchu resembles a v’yitnu.