"143" simply represents the number of letters in each word of the phrase, "I Love You."
The first recorded association of "143" with "I Love You" is from the year 1894, when a new flashing lantern was installed in Minots Ledge Light, a lighthouse located southeast of Boston Harbor. 💡 The light was installed after Lt. Fredeick Mahan of the Lighth House Board recommended all lighthouses have a numerical flash sequence, and 1-4-3 was chosen (with no apparent reason) for this particular lighthouse: one flash, then four flashes, then three. (Due to high costs, Minots Ledge Light was only one of two lighthouses that actually had numerical-pattern lights installed). Shortly after the new flashing light was installed, people started to recognize the connection between "1-4-3" and "I love you" and the lighthouse gained the nickname "Lovers' Light". The 1-4-3 light has since become automatic, and still flashes in the same pattern today.
💜Mr Rogers & #143 Once upon a time, around thirty-one years ago, Mister Rogers stepped on a scale, and the scale told him that Mister Rogers weighs 143 pounds... And so, every day, Mister Rogers refuses to do anything that would make his weight change...and every morning, the scale tells him that he weighs 143 pounds. This has happened so many times that Mister Rogers has come to see that number as a gift, as a destiny fulfilled, because, as he says, "the number 143 means 'I love you.' It takes one letter to say 'I' and four letters to say 'love' and three letters to say 'you.' One hundred and forty-three. 'I love you.' Isn't that wonderful?" (Thank you to Janet Gunn for sharing this)!