How much of that is due to how complex tickling someone is? Any repetition and you'll get used to it. Theres also that study proving even rats can get tickled. Which gets me wondering if crustaceans can get tickled as well.
Tickling was a torture method in ancient Rome and people can die from it. So no, suspectable people don't get used to it. Not all people have the same sensitivity however, so I can imagine it was like that for you.
"An article in the British Medical Journal about European tortures describes a method of tickle torture in which a goat was compelled to lick the victim's feet because they had been dipped in salt water. Once the goat had licked the salt off, the victim's feet would be dipped in the salt water again and the process would repeat itself.[...] However, it remains unclear if this method was ever used in practice as it is only described in the 1502 Tractatus de indiciis et tortura by the Italian jurist and monk Franciscus Brunus de San Severino – a treatise that actually cautioned against torture in general – and while it seems clear that Franciscus Brunus had not made up this practice, the issue is left open whether the inclusion in the treatise is based on hearsay, (reliable) eye-witness accounts, or personal experience.[...] This uncertainty does not preclude this anecdote from being repeated in popular culture, for instance during a 2013 episode of the British satirical quiz show QI.[...]"
Source: https://en.wikipedia.org/wiki/Tickle_torture#Historical_Anec...