Because that's what it's like, isn't it Dads? Before the holiday, you tend to arrive home at midnight, never speak to your family, break everything you touch and generally groan under the weight of your hideous deformities. It's a wonder you don't literally eat those children. But once you get on holiday, you slowly begin to participate in life, culminating with a sequence where your horns fall off (but you still manage to frighten the life out of a hotel maid), you stagger with your last ounce of energy into the sea… and then you emerge as a new man, a hunky fella whose family are happy to jump on, possibly including your wife (all these ads tend to hold out the hope of more sex with your wife).
Even though it's yet more 'Dads, eh? What are they like?' advertising, I'm not annoyed by the ad... I'm more annoyed that it's pretty much true (apart from the sex with your wife bit).