According to Osho, the urge to laugh simply means joy is blossoming; don't turn it into a problem by asking 'why.' The mind clings to questions and problems to avoid your empty, silent center, so it even doubts happiness. In closeness defenses drop, spontaneity returns, and laughter overflows. Trust it - no analysis, just allow the laughter and be.
You laugh because you're happy and relaxed; stop asking why and enjoy it.