Does expiation become obligatory for someone who vowed to pray and recite Surat Al-Kahf, then learned that vows are disliked, and sometimes feels that he recites the Surah because of the vow, knowing that he prays out of love and closeness to Allah and does not miss any prayer?

Fulfilling a vow, if it is an act of obedience, is obligatory and beloved to Allah the Almighty. Allah has praised those who fulfill their vows, as He says: "They fulfill [their] vows and fear a Day whose evil will be widespread." The Prophet, peace and blessings be upon him, said: "Whoever vows to obey Allah, let him obey Him." A vow of obedience includes prayer, fasting, Hajj, Umrah, freeing slaves, charity, i'tikaf (seclusion in the mosque), jihad, and everything similar in meaning. It must be fulfilled if its condition is met.

The feeling of the one who made the vow that he is reciting Surat Al-Kahf to fulfill the vow is commendable, and there is no expiation for a vow except to fulfill it as long as one is able.

The phrasing "I will do such and such" does not constitute a vow unless accompanied by intention, because it is an implicit rather than an explicit statement.

As for vowing to consistently perform the obligatory prayers, this falls under the category of vowing to perform an obligation, and it is not considered a valid vow according to the majority of scholars.