Researches shows that being with someone for a long time alter and changes the way you perceive and see the world. It also has its way of changing you.
Couples are husband and wife who had said the matrimonial oath and are expected to stay and love together till death part them. Couples who have stayed together for a long time shared a lot of experience and from what I researched and see around me, these five things happen to them.

• They start to have similar look

There is a saying in my local language which reads "TELE BA PE LARA OSHE O MA N DI OSHE", meaning " a leave that stays long on a soap will end up becoming a soap"
This is applicable to couples as they tend to have similar look as they grow old.
Also, Robert Zajonc, a Psychologist, found out in his study that there’s a very conspicuous reason that married couples who stay together start to look alike: They make use of the same muscles most times that over time they start to mirror each other and possess a similar look

• They possess a lot of inside jokes that others may consider not funny

Through study, couples who’ve been together for a long time are more likely to mirror each other’s body language. All of your shared experiences and memories inform your gestures, posture, and the words and phrases you use with each other.
It was found that people were more likely to copy each other’s eye gaze when they’d both heard the same background information before their conversation.

• Possession and development of private language

One of the first sign that couples are operating in sync is PRIVATE LANGUAGE. Communicating secretly accomplishes two things which are, it helps deepen your bond — romantic or platonic and it establishes a unique, shared identity.
This private language include everything in jokes and even nicknames. A research by Bruess shows that couples that use secret words and phrases are always the happier they tended to say they were.

• They start to sound alike

When two people spend enough time together or long-term couples, they begin to match each other’s speech patterns thereby matching each other in the basic rhythms and syntactical structures of their pattern of speech.
Couples tend to mimic everything from each other's accent to the amount and length of pauses he or she put between words and sentences.

• They stop self-monitor

When we’re in the company others, majority of us “self-monitor/self-censor”. By so doing, we try to satisfy the people that surround us by adapting our behavior to suit theirs. But things take a different shape when we’re with an intimate partner, we let go of this pattern of behavior and instead communicate with no limit, fluidly and naturally.
In other words, we stop having to constantly check ourselves before we speak. We’re more candid and more open.