The last section of today's Gospel doesn't seem to fit. It may have been a separate story, but the author of Matthew's Gospel sees it as a unit.
The first section describes a king's disappointment at the response to an invitation. This probably refers to God's sadness at people's indifference to Jesus. Others are invited in their place--all of those who will come to Christ.
The final part of the story presents the king's anger at one of the guests who isn't properly dressed. How could the guest be ready if he's been dragged in at the last minute? If you just came off the street, would you be likely to have an appropriate garment? The answer may be that God provides the right clothes for all of us: a suit of love. If we accept an invitation without caring about the guest of honor, why should we bother?