What is the ruling on inclining towards one aspect of religion, such as preferring Tafsir, Hadith, or Aqidah, and is the doer sinful?

It is recommended to increase one's pursuit of knowledge after learning what is obligatory, because it is among the best acts of drawing closer to Allah. The best field to increase knowledge in is the science of Tafsir (Qur'anic exegesis). There is no harm in specializing in a particular field of study over others, once the obligatory knowledge has been acquired. This is due to the varying natures of people. However, one should correct their intention, seeking the Face of Allah the Almighty and the Hereafter in pursuing knowledge beyond what is obligatory.