In this excellent poem we are presented with the effects of contemplating Grecian urns on the speaker. In the line you have identified, the speaker talks of how contemplating this Grecian urn has a profound impact on him:
Thou, silent form, doth tease us out of thought
As doth eternity: Cold Pastoral!
We can therefore see that viewing the Grecian urn is like contemplating eternity, and this in turn can lead to peace or to sadness as the speaker is then led on to consider the ephemeral nature of man's existence. Thus, if we allow the urn to "tease us out of thought," we would be in a state of mind that is beyond the material world of time and place, as we would be considering big, philosophical issues such as the nature of truth, beauty and our own transient nature as fragile human beings. The mixed message of this poem is that whilst beauty can be eternal, man definitely is not, and this can either bring peace and acceptance or sadness.