As other posts have indicated, Johnson may have had his good reasons to escalate the conflict in Vietnam by vastly expanding US support; however, the preceding president did not.
Kennedy, having witnessed warfare firsthand in WWII, and perhaps appreciating Vietnam's difficulties and correctness of such a conflict, had planned to reduce troops and withdraw. Although this may have been viewed as being "soft on communism," he may have realized that there was no real immediate threat to the US, unlike the Cuban Missile Crisis.
Certainly the arguments for being "hawkish" are well known; however, the real hawkish reasons may not have anything to do with halting Communism or expanding Democracy, but with who would control the oil off the coast of Vietnam (See 2nd link.)
Conflict and warfare are usually much more intelligible when political rhetoric is removed, and the grab for resources revealed.