Why would oil leak around a new oil filter?

There are several reasons your filter may leak. It might be the wrong filter. Did you oil the gasket before installing it. Did you over torque it (more than 1 turn after contact). Did you make sure the old oil gasket wasn't still on the engine block before installing the new filter. Could be a defective filter(bad gasket). In any case, better replace it to be safe, filters aren't supposed to leak. Better spend 3 to 4 bucks now than 3000 bucks later on a rebuilt engine.