This question is tough to answer.
To me, loyalty is one of the most important aspects of life, yet so many are disloyal.
I believe for one to truly express unconditional love, and loyalty, one must first love him or herself unconditionally.
Well there may be many reasons. Some of the reasons which I can understand are the following:
Materialistic: We have become more materialistic now a days as compared to past.
Compromising: couples are less compromising these days
Ego: we are having more ego issues as compared to our parents
Demanding: our demands now a days are very high.
Flexibility: we have less flexibility in our relations.
The most common reason behind most betrayal is greed especially with loved ones that are not loyal. This type of people will not think twice when choosing between you and what they can gain now and in the future.
In the era of Slave Trade, in African communities Fathers were selling sons, brothers were selling brother/sisters for money. Greed was valued much more then human lives. Other reasons may be fear, a good example is Peter and Judges betraying Jesus in the bible. Peter betrayed Jesus out of fear, fear of the unknown (he was afraid of what could be done to him) and Judges betrayed Jesus out of greed, for what he could get even though he may have thought Jesus would have escaped.
I will give disloyal people a very long rope...
Hope this Helps
There's a saying that I hold very close to my heart it says " betrayal doesn't come from your enemies but from your loved ones".
I've had my share of betrayal and in those cases I ask myself how could he/she do such a thing ?, I thought we were friends ?. After giving this topic a long thought I came up with the conclusion that people betray because they are selfish.
Think about it all from all the past and present examples like Judas Iscariot in the Bible, Marcus Brutus and our personal experiences we find out that it was all for thier own gain. Some people mask thier betrayal with the notion that they did it for the greater good to which I say "ok"
The black community has a saying. "Once a snitch always a snitch". So watch out for betrayers and backstabbers because they can also be a family member.