Why do we become irrational when inlove?

Love does not make anyone irrational. Sexual arousal does. You can love someone: mother, father without any sexual attraction and you can experience arousal without love being involved at all.
Irrationality is caused by the blood being redistributed in the body during arousal, The sexual organs of male or female both swells with blood, but this deprives the brain from getting enough blood/oxygen resulting in a sensation of intoxication.

It shuts down parts of the prefrontal cortex that have to do with reason and forethought. Hate does the opposite. It increases prefrontal activity and makes a person more analytical and cold in reason.

Hormones and disorientation from our previous time period.

Raging hormones and lust override logic..

i had the same thing happen. who knows?

just plain chemistry

Because love makes us out our guard down and neglect our senses