Defeat will be in the game but when that loss is against a small team that too in the final match of any tournament, it is definitely hard to accept. But it has to be said that when Sri Lanka team under pressure, Pakistan can make them look silly. As a result of which Sri Lanka was able to score big and finally they win the Asia Cup.
You are right .It is very difficult to accept that an experienced team like Pakistan will lose against Sri Lanka in the final match. Especially the Pakistan squad is more experienced than Sri Lanka. So our expectations from them were more. But the Pakistani batsmen's performance was very poor in the last 2 matches against Sri Lanka. Pakistan was defeated because of the poor performance of the batsmen.
Pakistan was looking overconfident in the match and they were considering themselves a superior team to Srilanka. If you do not take your opponent seriously they can beat you on any given day and this is what happened in the finals. Even, in the match before the finals, Pakistan lost to Srilanka and did not learn any lessons from that match too.