Why does attachment make us suffer?
Scientifically, if we have an attachment for someone, this will eventually lead to abhorrence. It is because:
When we are attached to someone, we naturally have expectations from that person.

When that person is able to meet those expectations — we are happy with them. But if for some reason the person is unable to do so, it upsets us; and we get angry and annoyed with them.

The intensity of our emotions is related to the level of attachment with that person. Higher the attachment, the higher the reaction!
Repeated failure by the person concerned who is unable to meet our expectations would lead to frustration, annoyances, etc. This is because we feel that we are doing so much and we are getting very little in return.

Why do we have a strong attachment towards only some people and not everyone?
Akram Vignan explains the science behind attachment.
When you move a magnet around a pin, what will happen? The pin would move along with it. And if we bring the magnet closer to the pin, the pin will immediately stick to it.
Because of the presence of an electrical body in the physical body, our body too has the property like that of a magnet. So, when it encounters atoms compatible with its own, the electricity within gives rise to the body’s magnetic property, and thus attraction occurs. Whereas with others, the atoms are not compatible and hence there is no such attraction; just like the pin is pulled towards the magnet, and not a piece of wood. This attraction causes attachment.

Even if you tell your body, ‘Do not go there’, don’t get attached’ it will still go. That is because the atoms within are being attracted. So, the body automatically gets pulled away wherever there are compatible atoms.
What gives rise to the compatible atoms?
It is our opinions that we have sown in our past life. Based on our experiences in our previous lives, we tend to form strong positive or negative opinions about certain people. These opinions get carried forward in the form of karmic seeds from past birth into the current birth where they sprout as an attachment (and abhorrence) for those people.
Now that we understand the reason behind attachment, what do we do to overcome this?

This sequence of attachment and abhorrence will continue until one attains Self-realisation. Self-realisation means to experientially realise that, ‘I am a pure Soul, separate from the body and mind.’ This realisation that ‘I am separate from the body and mind’ automatically detaches us from the effects of the body and mind. Thus, we are detached from all the suffering that the body and mind goes through.

The attachment gets replaced by pure love
Soul, our real Self, is pure love. Pure love is free from the attachment that leads to abhorrence, hence it is free from expectations. Pure love is void of anger, pride, greed, and attachment. It is love that is beyond opinions and expectations, and hence this love remains constant.

Moreover, when one attains Self-realisation, one experiences oneness with every living being because he now knows that every living being is really a pure Soul just like me. One experiences oneness and unity with the entire mankind.
Thus, Self-realisation helps us overcome our attachment issue and it frees us from all sufferings forever,it manifests pure love within which makes us experience oneness with every living being!

To know more: https://www.dadabhagwan.org/self-realization/

Author's Bio: 

Ambalal M. Patel was a civil contractor by profession. In June 1958, spontaneous Self-Realization occurred within Ambalal M. Patel. From this point on, Ambalal became a Gnani Purush, and the Lord that manifest within him became known as Dada Bhagwan. A Gnani Purush is One who has realized the Self and is able help others do the same. Param Pujya Dada Bhagwan used to go from town to town and country-to-country to give satsang (spiritual discourse) and impart the knowledge of the Self, as well as knowledge of harmonious worldly interactions to everyone who came to meet him. This spiritual science, known as Akram Vignan, is the step-less path to Self-realization