I assume that your question is asking why Shylock loans Antonio the money and why he makes this deal where the collateral will be a pound of Antonio's flesh. If so, the motive is Shylock's hatred for Antonio. Shylock is hoping that the ships will not come in and he will be able to hurt (and probably kill) Antonio.
As to why Shylock hates Antonio, that is less clear. Shylock himself says various things. He says that he hates Antonio because he is Christian. He says he hates Antonio for lending money without charging for it. He says he hates Antonio for no clear reason. All of these may be true. However, it seems likely that he hates Antonio because Antonio has treated him badly. This is the most likely reason for Shylock to hate Antonio. It is this hatred that causes him to set up the bond between Antonio and himself.
What motivates Antonio to accept the proposed terms? Perhaps Antonio and Tubal should become friends. The negotiations imply that Antonio and Shylock acknowledge that they have had difficulty getting along and will try to do better.