Poor communication
Incompatible life goals
Incompatible values
Meddling in-laws, ESPECIALLY those with incompatible values
Financial irresponsibility
Substance abuse
Unresolved issues with previous partners, including issues surrounding child-rearing after a divorce
Unresolved emotional problems from childhood abuse or a terrible family life growing up

These are just the ones I've personally had to deal with... and EACH ISSUE I listed has been the primary cause of at least one breakup.