Why are relations between Pakistan and Afghanistan so tense?

The Afghan Defence Ministry says it's carried out several strikes in Pakistan in retaliation for an attack last week. It's the latest act in a growing cross-border violence between the neighbours. Pakistan says it was targeting armed groups along its frontier. And says several attacks on its territory have been launched from Afghan soil – a charge Taliban officials in Kabul deny. So, does this mean border tensions between the two countries will continue to escalate? And where does this leave the 300-million people on both sides caught between the cross fire?