Why was Abu Bakr’s affinity with the Prophet the strongest?

The affinity Abu Bakr may Allah be pleased with him had with Allah's Messenger ﷺ is the fourth stage of affinity, which is called Nisbat e Ittahadi or associative affinity. The quality of Abu Bakr may Allah be pleased with him was Sadiq, the truthful, and of those who followed the Beloved Prophet ﷺ he was unquestionably the truest.

It is mentioned in a sahih hadith in Bukhari, as narrated by Abu Sa'id Khudri may Allah be pleased with him, that the Beloved Prophet ﷺ addressed the people and said that Allah has given option to a slave to choose this world or what is with Him, and the slave has chosen what is with Allah. Abu Bakr may Allah be pleased with him wept, and the Companions were surprised as to what had caused him to cry, as the Beloved Prophet ﷺ mentioned a slave who had been offered a choice. Abu Sa'id may Allah be pleased with him said that they later learnt (when the Beloved Prophet ﷺ passed away) that the Beloved Prophet ﷺ himself was the person who was given the choice, and Abu Bakr may Allah be pleased with him cried as he understood what the Beloved Prophet ﷺ meant. [Sahih al-Bukhari 3654]

At the time of al-Hudaibiya, when Umar may Allah be pleased with him asked a question, Abu Bakr may Allah be pleased with him responded just as the Beloved Prophet ﷺ did. [Sahih al-Bukhari 2731] This shows that Abu Bakr may Allah be pleased with him had the strongest affiliation with the Beloved Prophet ﷺ. It also proves that he was the best of the Companions, the most perfect of them and the most knowledgeable of them regarding Allah, His Messenger ﷺ and His Religion and the strictest of them in conforming to it. [al-Jawziyyah, Zad al-Maad]