It may be because the person apologizing is looking for acceptance from the other person involved. Apologies are not always accepted gracefully though and this is where the problem lies. When your significant other apologizes make sure to listen and thank them afterwards. Try to never make condescending remarks or give then disapproving looks mid apology. If you genuinely listen to their apology it will more likely be true and sincere as well.
Open apologies invite good communication, something every relationship needs and benefits from.