Until fairly recently, most corporate takeovers (i.e. one company buying another) were done primarily for business development reasons, for economies of scale, for access to increased distribution, and/or because the target company seemed inexpensive to the acquirer (though sometimes an outsider would wonder just what value management of the buying firm thought they saw in the target).
More recently, while those factors remain important, another impetus for international M&A (mergers and acquisitions), specifically U.S. companies buying foreign companies, is for the U.S. company to be able to become a foreign-domiciled company to avoid the U.S.'s destructive corporate tax code which, unlike almost any other country's tax code, taxes income earned overseas.
Thus, part of American drug giant Pfizer's interest in British drug firm Astra Zeneca is because if a merger were to happen, Pfizer would legally turn itself into a British company (while maintaining headquarters in New York and listing on the New York Stock Exchange).
Another reason that American companies are looking to buy foreign companies is that if the American companies "repatriate" their overseas profits, they face huge taxes on the money, so they figure "we might as well spend it on something outside the U.S." Pfizer reportedly has over $60 billion overseas. Why should they waste $15 billion or $20 billion (I don't know just what their effective tax rate is) by bringing it back to the U.S. when they can get the full value of it by just buying stuff overseas?
Similarly, Apple will soon be issuing a massive $17 billion in debt even though it has $150 billion cash. What's the catch? An estimated $130 billion of that hoard is overseas. So Apple will issue debt and use it to buy back stock in the United States rather than simply bring its overseas earnings home.
America's tax structure is counter-productive in so many ways, it's impossible to name them all. This sort of big-money international M&A is one of the most visible manifestations of it. Unfortunately, the reaction of most of the left, rather than trying to offer relief to the American companies, will be to try to penalize them based on the belief -- which must be challenged more aggressively and more frequently as not just inefficient but immoral -- that some part of every person's earnings and every company's earnings belongs to the government. After all, as the left's current wunderkind Senator Elizabeth Warren (Communist-Mass.) likes to remind us, "You didn't build that."