Why did Lord Rama break Shiva's ‘Dhanush’ from the middle

Before we discuss why Lord Rama broke the ‘Pinaka’ right from the middle, it is important to know why he had to break Lord Shiva’s dhanush in the first place. King Janaka wanted to find the right husband for his daughter Sita. And so, to decide who is the most suitable, he organised a ‘Swayamwar’ and put forward the condition that the king who is able to lift and string Lord Shiva’s dhanush will get Mata Sita’s hand in marriage. Kings, men and strong people came from all over the world in order to have Mata Sita as their wife, but unfortunately no one could string ‘Pinaka’. The swayamvar becomes the stage upon which the valour of the contenders is tested, but unfortunately none could do it.

The condition set forth by King Janaka could not be fulfilled by any and thus he became both angry and disappointed. While it was no ordinary task to string the formidable bow of Lord Shiva, the men who came to the Swayamwar had all declared themselves the greatest of all, a title which was soon going to be stripped off.
This divine Pinaka was filled with cosmic power, and could only be lifted by the one who had strength, righteousness, and the ability to uphold dharma. Seeing no progress, an angry Janaka roared that none of the Kings present in that Swayamwar should ever call themselves ‘Veer’ for they couldn't even lift an inch of Shiva’s Pinaka.

When King Janaka was angry with everyone, Lakshmana announced that Lord Rama could string the bow and the gathering went silent. Unlike others in that swayamwar, Lord Ram did not appear to be as powerful and this made everyone worried about his status! Lakshmana, Mata Sita, Sita’s mother and her friends, all prayed to the Gods to enable Lord Rama to string the bow with ease. The story goes that Mata Sita even asked the power residing in the Dhanush to lower its weight by putting it on the other people so that Lord Rama can easily pick it up and string it. And so, he approaches the divine bow, and the atmosphere is charged with anticipation.

The moment of Lord Rama picking up and breaking bow are summarised with the lines - ‘तेहि छन राम मध्य धनु तोरा। भरे भुवन धुुुनि घोर कठोरा॥’

It is said that the act of breaking the Pinaka happened in the same time duration as that of lightning. That is, the mere few seconds we see the lightning strike, in the same amount of time Lord Rama picked up, stringed and in the process broke the Pinaka from the middle.
It is also narrated that when Lord Rama broke the Pinaka, the sound produced by the snapped bow was so powerful that everyone around, humans and animals alike, lost their calm. The horses changed their directions and the force could be felt till miles and miles away from the breaking.

It is noteworthy that Lord Rama chooses to break the 'Pinaka' out of the three main dhanush of that time. Lord Rama used to carry the Sharanga, Lord Shiva’s bow was the Pinaka, and Lord Parashuram’s bow was the Ajgao. Each of these bows holds its own significance in Hindu traditions and represented different facets of the divine and their cosmic power. However, by choosing to break the 'Pinaka,' Lord Rama reaffirmed his connection to Lord Shiva, the supreme deity.