Do aortic dissections usually occur at the site of aneurysms?

Yes: If there is an aneurysm, there is a risk of dissection (in the thoracic aorta) or rupture (in the abdominal aorta). However, many aneuryms are small and will never dissect or rupture. To further answer your question, dissections do not occur spontaneously in normal aortas.

No: Aortic dissections can occur for a variety of reasons, including trauma, connective tissue disorders, hypertension, etc. What is required is a tear in the lining of the aorta, with blood leaking into the aortic wall. An aneurysm does not have to be present for a dissection to occur, but an aneurysm can develop from a dissection.