Astrological convention holds that harmonious aspects between charts are the significant factors for a positive relationship. However, it may be useful to have a blend of challenging and easy aspects for best results. For example, Venus and Mars have a great deal to do with sexual compatibility. Mutual trines and sextiles can make for an easy flow of energy, yet that might become boring over time. However, some harmony mixed with a challenging aspect, i.e. your Venus is trine your partner's Mars, your partner's Venus is opposite yours, can keep a level of dynamism that will continue to make sex an interesting subject for you two.
Couples tend to create their own little universe.
If both agree on something then it must be true. This can limit the development of the two individuals when their charts, or parts of them, are too similar. What's called compatibility may simply be shared neuroses. It is useful, then, that couples don't have all their planets align harmoniously. A little tension not only makes life interesting, but it helps keep perspective in the partnership.