Calling a top in the bull market is a way to go broke quickly or lose your job if you're a Wall Street strategist.
Big rallies usually find different reasons or a different combination of the old reasons to end. It's usually never the same.
That may be even more so the case for this six-year bull market as record stimulus from the Federal Reserve causes what appears to be excesses to be larger and last longer than during past tops.
Still, many strategists are starting to sound the alarm as past bear market signals begin to mount. Things like big merger activity and a record rise in borrowing to buy stock are giving many pause.