In the interest of full disclosure, my (strong) personal preference is for Hillary Clinton to be the next President of the United States (#ImWIthHer). But even absent this personal preference, at this point in the campaign there are innumerable reasons for Sanders to concede to Clinton in the race to the White House and not merely in the individual states in which she outshines him (sometimes very modestly as in Missouri, other places much more dominantly such as in Virginia).
Whether or not you like superdelegates (definition: an unelected delegate free to support any candidate for the presidential nomination at the party's national convention), they change the delegate math in a way that makes the nomination virtually unattainable for Sanders. While one needs 2,382 of the 4,763 delegates in order to win the Democratic nomination, Clinton currently has 1,119 to Sanders' 813. But among superdelegates, Clinton currently has 467 to Sanders' 27, making the delegate gap much larger. To be clear, this is not a case in which the superdelegates are going against the preferences of the rest of the delegate population. Clinton leads in both subgroups. But the superdelegate differential reinforces the upward climb that Sanders would face, an upward climb that is moving steadily from challenging to futile.
And yet he still has ample support, and there are arguments of representation in American politics that can be made in support of his continuing to represent those preferences (which admittedly from a policy standpoint differ only minimally from the preferences of Clinton, who emphasizes experience and pragmatism in implementation of a similar core agenda). There are also cases in which candidates can serve important roles to put on the agenda issue items that mainstream candidates will not, but maybe should at least think more critically about, or promote some dialogue in the media and among the voters themselves. Kucinich's campaign in 2004 serves as a salient recent example. And despite apparent determination to take the nomination battle to the convention, we have seen some rhetoric from Sanders that echoes the inclination to put issues on the agenda, to force discussion of issues, as opposed to battling for those ideas to be put into practice.
The problem is, when treating the race as hotly contested (and it is being hotly contested in a number of states, though keep in mind that New York has not yet voted), Sanders is -- as does any candidate -- making a number of attacks on Clinton. It has been argued that his becoming an "attack dog" is the only winning strategy he would have left in order to succeed. And that is certainly his right given that she is his opponent. But given the high probability that she will indeed win the nomination, battles on the nuances of policy and personal attacks leveraged among Democrats become ammunition come the general election. Sanders is in effect writing attack ads for the Republicans or shortening the amount of time they need to do on opposition research. And that isn't for the good of the Democratic Party. (Indeed, it has not gone unnoticed that some are attracted to both Sanders and Trump given their anti-establishment rhetoric, in addition to going after Hillary on the issues on which the GOP will be focusing going into the November general election). What it shows is that, like any human and particularly like any politician, he has self-interest and is acting on it, even to the detriment of the policies that he so vigorously defends.
Sanders has come much farther than many (even he) thought possible, and it was going to be a difficult road no matter what (not aided by his being a self-proclaimed Socialist, which is a label unlikely to play well among Blue Dog Democrats, independents, and moderate Republicans feeling that their party's candidates are out of step... also not aided by his failure to garner support among key Democratic constituencies such as the African American community, or the fact that he does not hail from a key state). He should feel proud of what he has accomplished, both in delegate counts and in promoting discussion of a progressive vision of what America can and should be. But that is where is should end, lest he help facilitate a change in presidential partisan control and in turn the (potentially far) rightward direction of American social, economic, and foreign policy.