There are three basic reasons for the number of denominations today. The first is there have been some major Theological differences. Not every teaching in Christianity comes directly from the words of Jesus. (That statement would take a book to completely unpack). Because of this there are sometimes where there are major differences in how two groups of Christians understand their faith. These differences lead to separations within the Church, the most notable of this type are the Great Split of East and West in 1054. This split happened because of the addition by the Western church of filioque (and the son) to one of the creeds. This might not sound like much, but it has a pronounced effect on how one expresses Christianity. There have been other times where such real controversies have caused divisions within the Church. For the most part, though the disagreement is serious and impacts the denominations, in the modern world such splits do not lead to ill feelings and often these denominations will work with one another on a variety of projects.
Unfortunately, there are times where pride gets in the way of Christian ministry and splits form from controversies which are petty and insignificant. Also, unfortunately these splits happen far more frequently than the ones with real differences. These are the separations which are regrettable and make churches seem foolish.
The vast majority of the reason for denominations is cultural differences. There is no theological difference between these churches, rather, they have different ways of doing things and stay separate for that reason. Methodists in Britain are distinct from Methodists in the U.S. simply because of such cultural differences, and the same is true for the Syrian Orthodox Church and the Greek Orthodox Church. Jesus did not give point-by-point instructions for how the Church should look rather he told us to “love God and love neighbor” and this can take on a variety of different forms. These different expressions of Christ’s community can be a wonderful thing and express the mosaic of human life and society.
For more information see
Justo Gonzalez The Story of Christianity vol 1 & 2
Bruce Shelly Church History in Plain Language