Relationships always lack stability. Here is why
Because relationship stability is a dynamic optimization problem. Let me put this in a mathematical way!
Here is the QuoraAnswer for this question
The essential components in a relationship :Sternberg in his theory of love states that to have a consummate love or marriage you need all the three components (Passion, Commitment , Intimacy)
Passionate love is based on drive. Couples in passionate love feel physically attracted to each other.
Commitment,is for lovers who are committed to being together for a long period of time. One can be committed to someone without feeling love for him or her,
Intimate love is the corner of the triangle that encompasses the close bonds of loving relationships. Intimate love felt between two people means that they each feel a sense of high regard for each other.
Where does the dynamics come in?
These three components determine the stability of a relationship/marriage. It is the time variance of each which causes the problem.
Intimacy and passion are oscillating functions (Too much of an increase in one decreases the other). Because Passion is based on Desire, mystery, thrill, enigma, But intimacy is based on security, openness , warmth.
Couples staying in long term, become very intimate but will lack that mystery they once had , it is due to this effect.
See this TED talk for more info: The secret to desire in a long-term relationship
Commitment is a constant factor which is a vital element of a relationship without which it is either a fling or a timepass. But it constitutes 50% of the stability. Commitment could be due to kids, just the bond of marriage or tradition.
You will see perfectly happy couples in platonic relationships with just intimacy and commitment. Or couples are crazily in love, fight a lot (passionate kind of love) and committed and stay together. What ever it is it is a combination of factors which gives the over all score.
Relationship stability = Commitment (Constant value-50% of weight) + (Intimacy +passion 50% of weight -More of (sin x +cos x) form)
This is the reason you see why stability is to be dynamically adjusted for a perfect relationship.
Even couples who are perfectly comfortable and intimate feel they lack something because of the lack of passion.
Those who are mad for each other feel the other person is not very trustable(Only passion and not intimacy)
Bottomline: Relationship stability is a dynamic optimization problem where at every point it is necessary for the couple to deviate and get back for long term happiness and it is one where commitment factor is at the core of their existence.