Syria is a country that is in the Middle East. There has been many problems with the Middle East including where the terrorists seem to come from. Since the U.S. and Russia are directly involved in fighting terrorism, they would be involved in Syria.
However, Syria has its own problems with the war that they are fighting themselves. U.S. and Russia find that they want to help the citizens and refugees by providing support so that the citizens do not get injured or killed. The Syrian government is known for attacking their own citizens. The journalists go over to Syria and produce the pictures that they take of the children and innocent people who are being injured and killed. This causes other countries to get involved.
The United States aims to defeat Isis and other terrorist groups in Syria. The United States and Russia work side by side to defeat the Islamic State in Syria. If they are successful, they could push their respective Syrian allies into negotiations toward a multi-ethnic, multi-sectarian, unified Syria.
Each side's positions are flexible enough to come to a compromise. The United States also wants to defeat or at least weaken other Islamist extremists among the anti-Assad rebels, as do the Russians. US and Russia want Syria to remain one unified country rather than being broken up into sectarian bits. Both the United States and Russia want Assad to go. They believe Syria will not have peace as long as Assad remains in power.