Answer: With no access to the ocean all trade has to pass through other countries. These countries can tax your imports or block them completely, and the same for exports.
Any military expedition has to pass through other countries. If those countries disagree with you they can hamper abilities.
Any water flowing through your country can be effected by upstream countries. This is still an issue for non landlocked places but I think its more of an issue if all your major water courses start outside your country.
Without trade any country would collapse, and a land locked country is dependent on it's neighbors for trade.