When astronauts think about their families or the unpowdered food back here on Earth, outer space does indeed permit them to tear up. However the zero gravity of space does not allow them to cry. That is, the tears are not pulled down the person’s face as they are on Earth.

According to astronaut Andrew Feustal who chocked up after he got something in his eye, the tear drops just conglomerate around the the eyeballs. This apparently produces a stinging sensation. Scientists do not know the reason for the stinging but have suggested that it may be because being in space renders your eyes very dry and large amounts of moisture applied all the sudden to the cornea are thus not tolerated. The funny/tragic thing is that a stinging sensation in your eyes actually produces more tears.

(Source)