Hi RocketQueen.
Because it's basically a cult that uses coercive techniques to keep people within the structure.
If he were to have a girlfriend who was not a witness then eventually it would be assumed that he was sleeping with her. This constitutes "fornication" which, if practiced unrepentantly, would lead to his disfellowshipping as a person willingly committing a "gross sin". Disfellowshipped persons should not even be greeted in the street.
The organisation promotes loyalty to God and his standards (i.e. their interpretation of them) by equating loyalty to the organisation as loyalty to God. Families are put in the difficult position of having to choose association with a DFed person or risk their own status as an approved person in the organisation.
Disfellowshipping is promoted as a positive thing which can help one to want to return to the fold. Compromising on association with a DFed person is seen as undermining the disciplinary process and disloyalty to Jehovah and his organisation.
Even if your friend tries to tread a fine balance and avoid being accused of a gross sin, his behaviour would be perceived as not meeting the mark and individuals, including member of his own family, may choose to shun him or minimise their association.
Your friend can just leave and avoid contact with the Witnesses ("fade") but at some point any "sinful" actions may catch up with him and he could end up being disfellowshipped. Having said that, as a single and young person this is far more achievable than for those even more ingrained in the structure and those with families.
Hope that helps a little!
