As an amateur watcher of the markets, I think market sentiment is a funny thing, and someone's personification of them as a bipolar guy who is either deliriously happy or depressed is pretty accurate in this way: Right now the markets are happy and are shrugging off any bad news. Someday, the markets will be depressed and shrug off any good news.
Why this happens, I've no idea. What causes sentiment to shift, I've no idea. It does seem, in my study of history, that the shifts are usually due to news items that don't really seem like they should cause a sea change. It also seems that these items are not usually ones that people were predicting. For example, the "Asian contagion" crash in the late 90's was not predicted by many. The mortgage-related crash in 2008-9 was seen by a few and probably obvious in retrospect, but the topic IIRC did not dominate the news headlines before things actually started unraveling.
As someone above posted, monetary policy can be a powerful force, and it does seem to me as though interest rates will probably need to be raised soon as inflation is likely to pick up soon. But I do like Yellen's data-driven policy even if it is less predictable.
I think the markets are high now because companies seem to be doing well and earning money. Why companies are doing well and earning money is more of a puzzle - labor costs are probably still lower than they otherwise would be as unemployed people return to the workforce and are just thankful for a job. Regulation changes and the possibility of tax reform, especially the corporate tax rate reduction and repatriation ideas, are probably driving stock prices up somewhat.
It could also be momentum. People with cash on the sidelines see the market going up for a few months, and they decide to join the party with some of their dry kindling, which results in the market going up more.
I like the high stock prices, but I do think we are beginning to form something of a bubble. How long it will last, how high it will get, and how bad it will be when it will pop are anyone's guess. I have my opinions, but as I said at the outset, I am just an amateur.