So I guess there is a lot to try and tackle here.
Stock markets in general are at highs in historical terms but there are also companies making more money than ever, the valuations of some aren’t outrageous - even Apple and Alphabet for example don’t have stupid PE ratios. So a lot of markets highs are driven by fundamentals. Companies like Amazon and Netflix are priced very highly for growth so they look like outliers, but we’re talking about correction, not recession or crash.
Interest rates are rising generally around the world which will probably pull investors back from risky assets, such as emerging markets or heavy growth stocks, so that is one downward pressure. The dollar is also high and looks like increasing in the short term, and a lot of emerging market governments have high levels of debt denominated in dollars, so if the dollar gets pricier compared to their own currency the cost of replacing this debt gets higher. So there is a downward pressure on EM markets. This poses a dilemma in some cases as the S&P500 is quite pricey as a whole, and EM markets therefore look like good value but the political and government risk is higher and you can’t ignore the debt levels!
So maybe un-loved developed market equity eg UK and Europe. Oh no - Brexit (hate that word), Italexit (hate that word even more), and general sluggishness of Euro markets is a bit of a disincentive.
Markets cycles are just that - cyclical. Things go round but at different speeds and I wouldn’t use the “oh it’s been 8-10 years, you know what that means” argument. However, we have had a great run the last 10 years because we started from low prices and quantitative easing artificially kept interest rates low, forcing investors into equities. Also, Apple, Facebook, Alphabet, Amazon are all great companies.
Will prices get corrected, yeah probably. Crash? Don’t know. Causes - political maybe (Eurozone or something-something-China), maybe oil (political still?) or maybe dollar related.