Now because most international laws as we have seen are actually natural laws that already exist in many of the countries globally, then it is obvious that many states will observe international laws since they are consistent with their national laws anyway as is the case with abolishment of slavery for instance; so this is one reason why states would voluntary observe such laws.
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The second reason is that many states currently are signatories to VCLT which is the body that has the mandate to create international law, by their virtue of being members this means that member states are obligated to observe international laws failure to which will lead to pre-agreed adverse repercussions. Finally, states are also motivated to observe international laws since it is in their best interest to do so; examples of such instances are bilateral and multilateral trade agreements.
But for the same reasons at times, states might actually not observe international laws because of common interest for instance especially where they conclude that observing certain international laws would hurt their interest. A good example is the US refusal to be a signatory to Hague international criminal court which it is afraid that it might be used to try its soldiers who might have committed human rights violation in places such as Afghanistan and Iraq.
Other reasons that might make state not to observe international law is because they are not member states to such ratification or because such laws that needs to be observed are in contradiction to their national laws as was the case in South Africa before the scrap of the apartheid system.