Faith. As in act of. For whether we like it or not, paternity is an act of faith. A mother knows full well if a baby is hers or not. A father only deduces so. Short of locking up the mom from conception to birth or asking for a paternity test, the father assumes the kid to be his. Not an excuse by the way but still a fact.
Absence. From the onset, fathers were often absent. When our ancestors roamed the plains of Earth moms stayed at the camp or near it to care for the young while fathers went on the hunt. Later fathers worked out of the home while moms worked in it, Only recently with the comforts awarded by modern life did this equalized to some degree. Moms have lives out of the house and dads are learning to play a different role, a new partition. That absence was not voluntary though and dads shouldn’t be faulted for it.
Today and tomorrow. On average, dads live shorter lives than moms. From going to war to “manly” stupidity and risk taking, from trying to shun sickness and weariness at the steel mill, men have shorter lifespans than women. That means that statistically, your father is likely to go first. Tell him, show him today that you love him because you may not tomorrow. My own dad has passed away and I’ve felt a bit more alone ever since. Today, it stings more than usual.
History. Essentially, fathers are the link to history just as moms are the link to existence itself. From your surname in most cultures and countries to traditional roles in the armed forces ( and wars ) or family business and of course genealogy, the father is the anchor. Changes in our societies may make this point moot eventually but the records since the bronze age are those of fathers and that will never change.
Experience. Moms teach by daily example, by their care, by their devotion. For all the reasons mentioned above, fathers have on average less time with their children and what is left is usually more often one on one relations. It is often in these moments that good dads will share what they learned : views on life and life lessons. Partially on account of scarcity, these will ( unjustly it may seem ) have more impact than the repetitions from mommy. This may seem unfair but it is not : each parent through its sex, role and responsibility impacts the child differently.
Respect. As said the Bible, respect your father and mother. Their job of raising you was almost entirely motivated by love and/or, at worse, by responsibility. What’s more they had to learn it on the fly, as it went! If you are still alive, it is in big part thanks to them!
Specifically, my own son called to wish me happy father’s day, being away. I asked him what is the central quality of a dad. He said it was to be funny. This may be solely a reflection on who I am and not on dads in general but if your father is funny, thank him twice?