Why do people buy stocks at higher price in merger?
There are kind of two answers here: the practical reason an acquirer has to pay more for shares than their current trading price and the economic justification for the increase in price. Why must the acquirer must pay a premium as a practical matter?  Everyone has a different valuation of a company.  The current trading price is the lowest price that any holder of the stock is willing to sell a little bit of stock for and the highest that anyone is willing to buy a little bit for. However, Microsoft needs to buy a controlling share.  To do this on the open market they would need to buy all the shares from people who's personal valuation is low, and then a bunch from people whose valuation is higher and so on.  The act of buying that much stock would push the price up by buying all the shares from people who are really willing to sell.  Moreover, as they buy more and more, the remaining people increase their personal valuation so the price would really shoot up.  Acquirers avoid this situation by offering to buy a ton of stock at a substantially higher, single price. Why is Linkedin suddenly worth more than it was yesterday? Microsoft is expecting to be able to use its own infrastructure and tools to make more money with Linkedin than Linkedin would have before.  In other words, they believe that the Linkedin division of Microsoft after the merger will be worth more than Linkedin alone was before the merger.  This synergistic idea is the theoretical foundation for mergers in general and the main reason people use to argue for a higher price. You could also argue that by expressing an interest in Linkedin, Microsoft may be telling us something it knows about Linkedin's value that maybe we didn't realize before because we aren't as smart and informed as the people on Microsoft's board.  But since it's Microsoft that's doing the buying in this case, I'm going to go out on a limb and say this is not the main effect.  Given Microsoft's history, the idea that they buy expensive things because they have money to burn is more compelling than the idea that they have an insight into a company's value that we don't.