The initial hostility between the two races in WC3 was so-so. Both sides made cock ups that wound up causing the other to become more and more aggressive towards each other.
The continued aggression post-Third War however is however simple Orc aggression. They had returned to a neutral understanding after working with each other and then the Orcs decided to continue invading even after they knew full well what they were doing. They know its their home but they don't care.
It's also disputable how much of the Orcs reasons are simply lumber given how they apparently have left much of it to simply rot.
Kaldorei easily at this point. Like Zael said the Night Elves have the stronger claim in Ashenvale whereas the Frostwolves mirror this in the Alterac Mountains.