When you had been asked to prove your relationship with your partner who can be either friend or fiancee or even spouse, whom you are absolutely comfortable / crazy with? What was your reaction such demand? I can surely claim that such demand must have validated the widening gap instead of cementing the relationship. Ofcourse we feel hurt by the people who matters us the most rather than some non committal relationships. But demanding the proof of the relationship is something extreme and even breaking point in many relationships, no matter how much intense the love is.
Every relationship is built of bonds that go beyond words, sometimes expressed explicitly but most of the times implied implicitly. Think of your close friend's bonds. Chances are very less that you might have proclaimed to him / her that you like them the most and after that they started moving closer. We unknowingly bond, realise that we enjoy other's presence and when both of you feel the same, the bond grows stronger, making others a beautiful part of our daily life and the whole process is natural, without any artifice . Certain incidents might bring the protagonists closer, some incidents make us see the others in a positive light but they just add the effect of the feelings. No equal relationship can evolve out of gratitude or lust. The people in relationship / friendship must be able to look into other's eyes, not look upto others.
So people who are in the serious relationships like soulmates or in love exist by beliefs rather than by proof. Imagine suddenly someone whom you considered as a soulmate asking you to prove the relationship by demanding that 'If you are my real friend, do this' else start doubting your intentions or activities. The protagonist obviously is hurt and a sort of frustration creeps in. Whatever looked as a beautiful past is now viewed with a different perspective. All the beautiful memories can do is just put a dry smile in the lips and a disbelief / cynicism in the outlook.
Many people see this demand to prove the relationship as cutting into their personal space. Mostly out of frustration people tend to withdraw either to protect their personal space or to avoid boring, sentimental conversations, which unfortunately strengthen the beliefs of the doubting partner. The relationship starts falling apart and part with bitterness. There can be numerous instances of tiffs and arguements but even one instance of demanding the proof is a doomsay for the relationship.
In this tension ridden world, everybody likes to have some soothing breathing space. The friendship and relationships are meant to contribute positively and cheer the sagging spirits. Until and unless you are a masochist, you won't prefer a cribbing partner. If you analyse your long lasting relationship, you'll find that the surviving one is what you never demanded the proof.
What if there is a element of suspicion in a relationship? As far as I had seen even an iota of doubt is explosive enough to burn the relationship if not worked on properly. Never go by hunch or impulse in a relationship. Whatever you see need not be the truth. Put yourself in your partner's shoes and sometimes to much of your surprise you might behave the same way. Anytime you go with statements like 'You seem to have changed, you seem to drift apart', the reaction will be initially sympathetical and if the situation continues, the partner actually changes or drift apart for above discussed reasons.
If the doubts are strong enough, just sit across and explain the incidents which made you feel so rather than bluntly saying that 'You have changed'. Many times the partner might have valid points that could clear the misunderstanding. If still you are not convinced, better take a break from the relationship. Most of the people love partners who can walk along in the lawns of life, not who emotionally ride on their back. So it is better not to be sympathetically attention demanding in relationships. Always keep the relationships light and lively.. you might find it lasting long without much of efforts to be so.