Verb paradigm in King James English for think
1 (I) think (we) think
2 (thou) thinkest (you) think
3 (he) thinketh (they) think
Infinitive: (to) think
These unfamiliar suffixes are applied in the same context that the -s suffix is applied in Modern English; for example:
During the Early Modern English period, the 2nd person singular suffix disappeared and the -th suffix in the third person was replaced by another suffix, -s, which spread from dialects in the northern parts of the country. Other conjugations, such as -e in the first person singular from Middle English, had already been lost.
This sort of change is known as paradigm leveling. There is no particular reason per se that this kind of change happens, but it is not uncommon in the languages of the world.