At the beginning of scene 2, Act II, Launcelot mentions one of the reasons why he should leave Shylock's service. In his monologue, he makes it clear that it is not an easy decision and is something that sits on his conscience. He believes, however, that Shylock 'is a kind of devil' and later emphasizes this by stating that 'the Jew is the very devil incarnal,' when he actually means incarnate. The implication is that Shylock is very difficult person to work for since he makes Launcelot's life a misery. Launcelot resolves that he will follow the advice of the fiendish aspect of his conscience and run away.
Later in the scene he provides another reason for his desire to abandon his employer. In conversation with his father, Launcelot Gobbo, who wishes to present a gift to Shylock, he states the following:
My master's a very Jew: give
him a present! give him a halter: I am famished in
his service; you may tell every finger I have with
The reference to 'a very Jew' in this instance, has a derogatory connotation since it suggests that Shylock is stingy. His statement clearly reveals a stereotypical and cynical perception of Jews. Launcelot states that Shylock does not feed him properly since he is famished and has grown so thin that his ribs are as clearly discernible as his fingers. He wishes to be in Bassanio's service because he will provide him with 'rare new liveries.' This furthermore suggests that Shylock has not clothed him suitably and has deliberately not provided him with a new outfit for quite some time.
Later in the scene, when Launcelot tells Bassanio about his plans, he also mentions that he would prefer being in his service since Bassanio has the 'grace of God' whilst Shylock has only enough. The implication is obvious: Bassanio has more than enough generosity and kindness to give to everyone, whilst Shylock's is just enough for himself - he has nothing left to share.
If one considers all Launcelot mentions, it becomes quite understandable why he is so desperate to leave.
It is quite ironic, though, that Shylock later, in scene V, tells Launcelot that he will not have the opportunity to greedily stuff himself as full of food when in Bassanio's employ as he had been whilst working for him.