C-162 pilot reported a cabin entry door opened in flight. Pilot returned to departure airport and landed.

Narrative

At around XA40 Z I conducted a complete aircraft preflight inspection in accordance to the CE-Skycatcher M162 POH; Section 4; pg. 4-4 to 4-24. (4-6. Cabin Door - CHECK [security and condition]) The doors were checked for condition and security; and everything was found in normal operating condition. Just before takeoff I once again checked the doors on both sides as per CE-Skycatcher M162 POH; Sec. 4; pg. 4-18 (3. Cabin Doors - CLOSED and LOCKED) They appeared both to be closed and latched. I closed my door; and the passenger closed the door on his side as I instructed him. Furthermore; I told him to give the door a forearm 'bump' with his shoulder as I was taught by my instructor pilot to ensure closure. The takeoff was uneventful; with normal performance; airspeed; and engine indication. During the climb phase up to pattern altitude the flight was normal; with no noises or vibrations from the doors. How it happened: The downwind was established at the local pattern altitude for ZZZZ as the airspeed increased and power reduced. Suddenly; a loud blowing noise was felt I immediately noticed my door (Left-hand) had opened and broken. My first reaction was to fly the aircraft; the handling was somewhat difficult in my opinion (never had this happened before); and I was experiencing significant drag caused by the hanging door. At one point I added full throttle because the aircraft was losing altitude at a high rate. At this point; I contacted the tower and [requested priority handling]. The Tower cleared me to land on Runway X. From that point; onward the approach and landing were uneventful and safe at XB22 (Z) local. In retrospect; I can't determine how this event occurred. I have several assumptions; first I did not secure the latching mechanism properly; although I emphasized the importance of locking and securing the door to the passenger which included the forearm bump. We both did it. Second; the latching mechanism was probably locked but due to tear and wear and combined subtle airframe or engine vibration; it unlocked itself. (As a pilot with little experience I may not know how to assess that issue). Third; maybe the door alignment may have allowed airflow over the fuselage to force the latching mechanism to fail; case in point the event occurred while the aircraft was accelerating in the downwind phase. Or it failed as would any mechanical component. After this experience; I will be more alert to checking the door latching mechanism more thoroughly during preflight. I will include two events of opening and closing to ensure it is operational and it feels like a tight latch. I will continue to use the checklist as taught.