From what Meursault tells us, he seems to be drawn to Raymond because Raymond is a fellow outsider. According to Meursault, Raymond is a warehouse worker and "not very popular." Furthermore, Raymond seeks out Meursault's company. This saves the passive Mersault the effort of reaching out. As Meursault notes, Raymond stops by his place to talk and doesn't stay too long, which puts him in Meursault's good graces. Raymond also invites Meursault to dinner, offering him sausages and wine. Meursault accepts on the basis that it will save him the trouble of cooking a meal. Raymond also invites Meursault to the beach. Overall, Raymond makes the friendship very easy for Meursault, who feels he has no reason "not to" let it unfold.
Beyond his passivity, Meursault appears to have something akin to a liking for Raymond. Meursault states, for example, that he enjoys listening to him talk. He notes that he "find[s] what he has to say interesting."
We might also suspect that Meursault is, beneath his passivity, attracted to Raymond's active and energetic engagement with life, which is so different from his own. Raymond, a foil to Meursault, exhibits all the passion that is missing from Meursault's character. Raymond acts forcefully and purposefully, even if his actions are often cruel, such when he punishes his girlfriend or wants to shoot the Arab. By pulling Meursault into his drama, Raymond adds the interest and color that are lacking in Meursault's life.