Why does fiar Lawrence not believe romeo is in love with Juliet?

He is afraid that they are moving too quickly in their relationship, as before he marries the two of them when he is talking to Romeo for the fist time, he tells him "Wisely and slow, they stumble that run fast." This basically means that bad things will happen if they take things too quickly, and he was exactly right in his advice/prediction. There are some other times that he mentions taking their time, but of course they don't listen and die in the end.