Christ is in heaven, with a glorified body. Glorification does not destroy what is essentially human. A glorified body, thus, has a mouth, eyes, ears, and all that. There is more to it than that. But what normal human beings have, Christ has in heaven. But there would be something unnatural, something lacking in the glorification, if Christ had a mouth and ears, but there was no fellow human being there for him to talk to. Our bodies exist in large part for interaction with others (that our bodies are signs of our being-for-others is a central insight of John Paul II's theology of the body). A single embodied human being just does not make all that much sense.
But if there is a fellow human being in heaven for Christ to be in human communion with, then who is this fellow human being? It is right and proper to confer honor upon our parents. Thus, his mother is a very plausible candidate. There would be something unfitting about his conferring that honor on someone other than his mother, and not on his mother, given the commandment of filial devotion, unless his mother were not a disciple of his—which, the Christian tradition insists, she was. (Indeed, I think it is right to read the New Testament as presenting her as the paradigmatic disciple.)
Objection 1: This argument proves too much. For a glorified body also includes reproductive organs, and so a parallel argument would show that to live a full, glorified human life requires reproduction.
Response: Probably, reproduction is only a natural function of a human being for a limited portion of the human being's life. Moreover, there is a way in which we can transcend the physically reproductive life through spiritual reproduction—through devoting our lives in celibacy to spreading the Gospel. But some form of bodily interaction with other human beings—whether through talking or hugging or just looking in another's eyes—seems essential to a naturally fulfilling human life at just about all its mature (and maybe even immature) stages.
Objection 2: The need for bodily communion is satisfied through Christ's giving of himself to us bodily in the Eucharist.
Response: Christ's giving of himself to us in the Eucharist does not seem to make use of any of the natural faculties of his glorified body. There is a kind of natural bodily communion with others that is called for.
Objection 3: Mary survived at least some time past Christ's ascension into heaven. So if Mary is the only one assumed into heaven, for a while Christ was bodily alone in heaven, only surrounded spiritually by the souls of those he brought out of Sheol.
Response: Maybe then we have to say that more people were assumed bodily into heaven. Moses and Elijah are good candidates on Scriptural grounds. But the argument that it would unfitting for Mary not to be assumed as well if anybody is, given the special honor to be paid to parents, still remains. Or, maybe, we should say that there is nothing deeply unnatural in a human being's being alone for a while, even for a couple of years. But to be alone for a significantly greater amount of time would be unnatural.
Objection 4: Maybe time runs at a different rate in heaven, and it'll only be five minutes of heavenly time between Christ's ascension into heaven and the Last Judgment.
Response: Could be. I think such a difference in the rate of time, though, weakens the way in which Christ is in human communion with us. But I acknowledge that the different-rate hypothesis is a viable one, and hence weakens the argument.
Fittingness arguments, like the one I offer, are not meant to be conclusive. But they do increase the probability of the claim. Or, at least, the argument could help explain why it is that the doctrine of Mary's assumption is not something weird, unexpected and ad hoc, if the doctrine actually can be seen as helping to solve a genuine problem, the problem of Christ's bodily aloneness.