Brain aneurysms develop in a weak or defective artery wall, though the exact cause of this is not yet fully understood. It could be a degenerative process, where factors such as increasing age and high blood pressure play a crucial role. It is also often linked to atherosclerosis, where arteries become hardened due to a gradual accumulation of fatty deposits on the artery walls. Brain aneurysms typically form in the main arteries that run under the brain and the base of the skull, or at 'forks' in the arteries where the walls are weaker.