People tend to forget that our president-elect is quite a formidable businessman. Business is an industry in which relationships, networking and negotiation are paramount. I doubt that Trump is unaware of the necessity of maintaining positive relationships with our foreign countries as well as are the staff he chooses to surround himself with. Foreign relations are such an essential and inevitable factor in our world today, no president could ignore it even if they wanted to. Besides, maybe Russian sanctions are not a top priority for our nation at the moment.
President Obama is upset with the outcome of the recent presidential election. Like many other people, he wishes to make excuses for the outcome and discredit the results. If the alleged Russian hacking had worked against Trump, he would be pushing for harsh actions. He takes foreign relations as seriously as any other politician.
Trump isn't taking these issues with Russia seriously because he knows he had a part in the problem and that he benefitted from it. He isn't going to speak badly about Russia or their leader, as those are his allies. He knows that he needed them and will probably continue to. Trump cares only about himself, and couldn't care less about foreign affairs unless he somehow benefits.
Donald Trump cannot be taken seriously on any matter, because he is a celebrity clown who has somehow found himself in the White House. The next few years of his office are going to be an utter disaster. If he weren't the president, it could be funny, but he could very well start a war this way.