Is it really that hard to know a person?

I have been watching this show, 'Emotional Atyachar'. For those who don't know about it, the producers of this show perform a kind of spying operation and show us its footage. The operation is carried out to check the fidelity of a BF/GF, upon the request of doubting partner. It is a reality show. Due to its subject and shooting style(with hidden camera) and many other reasons, I liked it. Now bollywood has made a film on similar subject, 'Love Sex aur Dhoka'. Keeping its entertainment part aside, one question is nagging me all the time. Is it really that hard to know a person? Do you really need to do a spying operation to check one's fidelity?