It’s one thing to eagerly set out to do a mitzvah that comes your way, but it’s quite another to actually do it, then complete it. For, pity the poor human spirit: it so easily grows weary and so quickly comes to be bored, disillusioned, or distracted.
Who among us hasn’t set out to help someone move across town, for example, and then quickly come to be overwhelmed or overwrought by all the actual work involved? Who hasn’t meant to give more tzadakah (i.e., charity) then been stopped in his tracks by the thought of actually paying out the money? But there’s the rub. Because being kind and charitable literally means leaving your self and your own, and doing favors or giving money.
And doing it in the spirit of “enthusiasm”, the subject at hand, means doing it “not … because you’re anxious to unburden yourself of it,” as Ramchal term s it, “but because you’re afraid that you might not merit completing it” — that is, you’re beside yourself with the idea that you might get there too late or not do it till its end, while some luckier soul might, and you would have lost an excellent and lovely moment of goodness.
As we’re taught, “all of the actions of the righteous are done hurriedly”, i.e., they set out to do good excitedly, energetically, and with sparkling eyes. “They wouldn’t allow a moment’s delay either in the starting or completing of a mitzvah” (Bamidbar Rabbah 10:5)
As Ramchal sums it up, “The person whose spirit is aflame in the service of his Creator will certainly not be lackadaisical when doing mitzvahs. His movements would be as quick as fire, for he could not be at rest or still until he would have absolutely completed the task.”