In the poem ‘A Psalm of Life’ the poet has expressed his philosophical ideas and an optimistic view of life. The above quoted line comes in the very first stanza where the young man (the speaker) dismissed the claims of pessimists about life being “an empty dream”.

So, given the context, the speaker possibly mean, things are not what they seem when we are holding negative thoughts about anything or not taking it seriously. This is also true for the situations when we are too happy or too sad. That is why the speaker also says ‘Not enjoyment, and not sorrow, / Is our destined end or way;”