Of course the right conditions are indispensable but, beyond that, you need to actively tend your relationship.
This involves three things which I like to compare to lawn care:
1) Locating and addressing your “issues”. This is the weeding part, and the bulk of what I teach couples in therapy. Unfortunately, most couples do this aggressively and almost exclusively, leading to an immaculate, though usually bald, lawn. Even when it is well done, however, weeding alone is not enough.
2) Doing things together. This is the seeding and watering that strengthens and builds your “turf”, the je ne sais quoi which defines you as a couple. You can do this by going to the movies or out for dinner, playing sports together, or even just sitting around and talking.
3) Fertilizing. Take a few minutes each day to tell your partner what you love and appreciate about him or her. By telling each other what you love about each other, you give each other something which makes you feel good and just plain deepens the good feelings between you. This enriches your relationship immeasurably, at very little cost, making it a really outstanding and beautiful experience.