After the Bangladeshi War of Independence in 1971, Pakistan only recognised Bangladesh three years later after coming under pressure from other muslim countries. Relations then thawed with the then Prime Minister of Pakistan, Zulfiqar Ali Bhutto visting Bangladesh and paying homage to the war memorial in Savar Upazila and the then Prime Minister of Bangladesh, Sheikh Mujibur Rahman, widhsrawing bans on a number of pro-Pakistani organisations.
Relations improved even further under Prime Minister Ziaur Rahman of Pakistan and Prime Minister Mohammed Ershad of Bangladesh when Pakistan became more distant from it's allies, China and Russia and symbolised by a gift of several squadrons of F-6 fighters to the Bangladeshi air-force in the late 80s. Since then five Pakistani heads of state have visited Bangladesh. In particular, Zia al-Haq stating, "your heroes are our heroes", after visting the National War Memorial.
Bilateral relations go back some time, with both Pakistan & Bangladesh being founding members of SAARC founded in Dhaka in 1985. Many other bilateral agreements on trade and culture have been signed since.
It was probably the establishment of the War Tribunal on the 1971 genocide in 2013 that led to ties cooling when Abdul Molla was executed for his part in the war, with the lower house of Pakistan govenment issuing a condemnation calling the execution, "politically motivated." Bangladesh asked them to remove their senior diplomat, Farina Arshad on spying charges in 2015. Pakistan retaliated the following year by asking Bangladesh to remove their senior diplomat, Moushumi Rahman gor "anti-state activities".
However, since Imran Khan became prime minister in 2018, relationships have considerably improved with Pakistan recently revoking all visa requirements on Bangladeshi citizens visiting Pakistan. This follows a personal call by him in July 2020 to Prime Minister Sheikh Hasina after Bangladesh announced a foreign policy of "friendship to all, malice to none."
Why is Pakistan eager to strengthen bilateral trade ties with Bangladesh?
They were already improving ties, when these were chilled by the establishment of the War Tribunal in Bangladesh. With the election of Imran Khan as Prime-Minister of Pakistan three years ago and with Sheikh Hasina signalling a policy of friendship, relationships have resumed on an upward trajectory.