Why are the classification systems changing every now and then?

Millions of living forms exist on the earth that include plants, animals and microorganisms. Out of them, many are known and have been classified by the scientists under suitable groups, but on the other side, many new species are continuously being discovered. Many of these newly discovered species have a new set of characters and to classify them, either a new system of classification should be developed or an older one should be revised. This brings changes in the existing system of classification every now and then.