How's this for a reason why the bull can last longer.
Europe is actually getting its act together and growing earnings and GDP. Euro earnings are still ca 30% below 2008 peak whilst US already exceeded it. Largest US companies will get some of that growth, weakening dollar will also improve USD earnings.
I'm not saying US market can't go higher but:
1. if EU and especially EM markets are relatively better value then the money will flow there, not to US markets
2. Europe is getting its act together? Well I'm in EU and I can tell you that within EU only Germans are doing well and CEE. The rest of it is still a mess, just nobody talks about it anymore as it's old news
3. there is so much positive future already priced into US markets including the fact that the world economy is going to get (finally) better, yet it is the slowest recovery...
4. if the US economy is well and about to get even better, then FED will need to increase funds rate, so how do you come up with the "weakening dollar" statement?
As I said before, everything is a positive news or story for the markets. Since the markets climb the wall of worry, find me the reasons to worry please. If we can't find reasons to worry then either everyone is in by now (except algos) or we are about to see some irrational exuberance.