When I open my mouth my jaw goes sideways instead of straight up and down. Why is this?

The reason your jaw goes sideways when you open is the articular disk on the side your jaw moves towards is locked out. This is called a closed lock (not lockjaw, which is a medical condition caused by tetanus). The articular disk is a structure that is part of the temporomandibular joint that enables the mouth to open wide by translating or sliding along a path within the jaw joint (articular eminence). When the disk is prevented from doing so the jaw becomes locked on that side. If the opposite side can translate, then only that side is able to open properly and the locked out side remains stationary. The result is the mandible will deviate towards the side that is locked out. Deviation of the lower jaw upon opening is another diagnostic sign of a TMJ disorder. Long story short - you have a TMJ disorder and need to find a dentist who knows how to treat it (not all do) but avoid oral surgeons and traditional orthodontists, trust me.