I also wish to remember the "table fellowship" concept and the illusion of a "ritual purity." In Jesus' times, whoever was invited to your table was considered either friend or family. This created a bubble of people who were unliked. Jesus Christ opened up to them and ate dinner with prostitues, tax collectors, and various other disliked professions. This violated the "policy" of ritual purity. These people were dirty humbugs and scoundrels. Because of this, they were considered "unclean", ruining society's view of them.
I think that the reason why man discriminates is because, although we were made in God's image, He gave us free will, and from there the worse side of human nature got to us. We couldn't resist temptations and the urge to discriminate was overbearing because we needed to pump up our egos.