Rashi explains this sign shall be for you and not for others. Hence we may derive that the blood was smeared on the inside of the house.
The principal reason for smearing the blood on the inside of the threshold was for the Jew to comprehend the importance of self-sacrifice in the privacy of his home. He must concentrate on the inner dimensions of his personality. The essence of the Jewish act is not the one performed on the public stage, but the one performed on the inner stage, when the audience is only Hashem. The only audience to which a Jew should attach significance is the audience of Hashem. Inner heroism and self-sacrifice is the hallmark of a Ben Yisroel.
Another lesson may be learned here. Often we will attempt to help others in the fulfillment of Torah and mitzvos, even at great sacrifice to ourselves. This may be at the expense and the neglect of our own families. We will sacrifice our time and energy for others, but will we find time for our personal study and self-development? The Torah enjoins us to establish in our homes Torah sessions for ourselves. We must be aware of our responsibilities to our own children, to guide and encourage them ourselves, not by proxy through tutors. We are obliged to do for ourselves and our families that which we so readily do for others.
