Shylock himself gives Jessica (his daughter) the reason why he has decided to go to supper with the Christians:
...I am not bid for love; they flatter me:
But yet I'll go in hate, to feed upon
The prodigal Christian....
He admits that he does not go out of love because the only reason he has been invited by the Christians is to humor him. Their supposed desire to gratify him is only because he has agreed to lend them money. He therefore only goes because of his hatred for them. He will indulge them because he would feed on their food, and he will find pleasure in seeing how wasteful they are. Shylock is obsessively stingy and materialistic. To see the Christians' flagrant and excessive imprudence, contrasted to his thrift, would provide him much joy, for it means that he would always, in a materialistic sense, be their superior.
His initial rejection of Bassanio's invitation is also clearly explained by him:
Yes, to smell pork; to eat of the habitation which
your prophet the Nazarite conjured the devil into. I will buy with you, sell with you, talk with you,
walk with you, and so following, but I will not eat
with you, drink with you, nor pray with you....
His sarcastic remark at the beginning of the extract indicates Jewish belief that pork is not kosher. He finds it despicable that Christians could ingest meat into which Jesus banished a demon. His disgust for this particular dining habit symbolizes what he thinks of them. He states that he is prepared to have limited and superficial social contact with Christians but that he does not desire any deeper association.
The word "prodigal" means wasteful, excessive, uneconomical, imprudent, etc. This signifies carelessness on the part of those to whom the word refers. Shylock believes that Christians seem to take things for granted and are wasteful. They, unlike him, wantonly spend or give away their possessions. He actually takes pleasure in what he believes is a despicable habit, for it gives him an advantage over them.
This is obviously the case in this situation, since they have approached him, someone they despise, for a loan. It is profligate spending that has put them, especially Bassanio, in such an embarrassing situation, which is clearly a source of much pleasure to Shylock.