This is how a relationship can flip in one moment. And agreed, someone else triggers the love emotion, which is why we're all on this forum. This love emotion is irrational and that's why it's called a bomb-drop. This love emotion is stronger than any bond, 10, 15 or 20 years of M can build.
Nope, it's not the "love" emotion, it's infatuation. The MLCer only thinks it's love, but it is really not. Love does not happen in an instant, it grows from the tiniest of seeds and takes years to flourish. Again, the MLCer is a mentally compromised individual, that has his/her judgement clouded. This is why they often fall prey to the lowlifes. One would think they would trade up, but as it is pretty much always the case, the affair down. This is why the LBS is so confused. The LBS mind is somewhat clearer than the MLCer's so we, that is why this is mindblowing to us and also kind of an insult, because we keep wondering why were we replaced by the dirtbag.
The dirtbag usually has the sweet talk and they know just what strings to pluck to get the desired reaction. And the MLCer is convinced that they found the love of their lives and they discard their old life completely in favor of the dirtbag. Very often the dirtbags are very disturbed individuals, exhibiting a lot of traits of sociopaths and psychopaths. 99% of the cases the MLCer realizes (it might take them a couple of years) that the OP is a dirtbag, but very often the LBS closes the doors to reconciliation.
