Pesach—the festival of redemption and freedom—is completed only on Shavuot, which is the festival of the giving the Torah.
Thus, Pesach without Shavuot is incomplete and lacking.
And, in the same way, Shavuot needs Pesach in order to have a foundation in real life.
The two festivals are interconnected.
Whoever severs them remains with a partial entity, with only one aspect of things.
–Rabbi Adin Steinsaltz
From On Being Free, P. 28, by Rabbi Adin Steinsaltz