The most outrageous act of kindness in a relationship is consistent, day to day support. Having not felt this or received it or had it for so many years, when I finally did, it completely changed my life. Of course trust, loyalty, companionship, compatibility, lust, respect, interest etc. are part of this; but the deep sense of support that can be gained from a trusting, loving relationship can do more to enrich your life than any diamond ring, expensive vacation or country home.
As any parent knows (or any grown-up child, for that matter) kids need consistency. They are not going to feel stable or secure if their parents act differently depending on the day, the hour or their mood. Coming home in a fit of rage one day or forgetting all about dinner and then cooing and cuddling the next is not going to help a child develop a solid sense of self worth- quite the opposite; a child will begin to expect the worst and dread the good times; they will fear the loss of love at any moment and remain cautious, untrusting.
Well, these truths are bound up with adult relationships as well. Grown ups need consistency in their relationships in order to feel safe, secure, to continue to grow as people, to develop their own "love muscles" and become as giving and content as possible. Knowing that you can share your thoughts and feelings with your partner without getting into trouble about it, being misunderstood, getting into a fight or a bickering session, without a lot of hostility or judgement can provide more good, more healing, more bonding than any material item.
When couples who have had good, long relationships talk about the way their relationship changes over time, what they are essentially saying is "I've grown and changed, they've grown and changed, and we've trusted and supported each other enough to allow these changes to occur."
The simplicity of this is taken for granted in much the same way that small acts of kindness can be perceived as weakness or taken for granted. Yet small acts of kindness, daily support and trust, openness and a lack of hostility are the building blocks for any real profound relationship.