I think Jesus speaks so strongly against divorce and remarriage because they are not part of his plan of salvation, it is evil working, spliting, dividing people, "desintegration". And also because marriage it is a serious bond held between a couple and before God, the promises of marriage are made to each other, but above all, they are made to God, so when people divorce they are above all breaking their promises to God. Marriage was not made as a matter to do and undo later, like tide and untide shoes, it can not be like that, because marriage it is the foundation of family, what vital importance Jesus grant to it.