First, we must note that it is not necessarily correct to say that Sunnis typically dislike Sufis. This varies widely depending on the country in which the Sunnis live. Many Sunnis accept Sufis as true Muslims.
Where there is discord, it is largely due to the feeling that Sufi teachings are innovations that are not true to the Quran or to the teachings of Muhammad. Even more importantly, they feel that Sufism is not sufficiently monotheistic. They feel that Sufis believe that God is present in too many things. The Sufi emphasis on becoming one with God and experiencing God in this lifetime is said to verge on pantheism because it implies that all people and even all things can be part of God and God can be part of them.