Marriages are hard work and it takes much more than love for them to survive the test of time. While some marriages break down as soon as challenges arrive, others thrive and become stronger over time. But, what are couples in happy and healthy marriages doing differently which sets them apart from the rest? Dr. Jacqueline Olds and Dr. Richard Schwartz– couples therapists and associate professors of psychiatry at Harvard Medical School– who have been married for roughly 50 years recently shared their insights with CNBC. Here we list their advice on the five habits people in happy marriages avoid at all costs