Why does the Church teach abstaining from meat on Fridays during and outside of Lent?

The reason for not eating meat is twofold:

1. Eating meat is considered a luxury item, so by abstaining from eating meat, we are making an act of penance.
2. Since meat has blood in it, by abstaining from eating meat for our nourishment, we are saying that our only true nourishment is Jesus' Body and Blood.

The reason why Friday is selected for the penitential act of abstaining from meat, is because Friday commemorates Christ's crucifixion. By doing the penitential act on Friday, we remind ourselves that we are sinners, and that Jesus died for our sins.