From the Easter Vigil through the Day of Pentecost (the entirety of the Season of Easter), we have the option to end our Eucharistic Services by adding to the Dismissal "Alleluia, Alleluia."
Many churches I have attended, and some I have led, don't like the limit the use of these Alleluias to just the Season of Easter. Some have even defiantly added their Alleluias, even when the deacon, or other clergy, dismissing them doesn't add them.
Alleluia means "Praise God", and doing so is never a bad thing. However, there are other parts of the service where we add "Alleluia". The principal Fraction Anthem used following the silence after the Breaking of the Bread has the option to add an "Alleluia" at the start and at the end (and it is mandatory to do so during the Season of Easter).
The only time an "Alleluia" is not allowed is during Lent. This allows us to live into the more penitential nature of Lent. It sets this season apart.
The Alleluias at the Dismissal are similar for the Easter Season. They set Easter apart as a season of joy. They help us realize this Season is special, and they help us live into our remembrance of the Resurrection all the more.
Once the Season of Easter ends, it is appropriate to end the practice of Alleluias at the Dismissal. Doing so helps us set Easter apart. It doesn't mean we praise God any less in the meantime. It just means we realize the importance and profoundness of Easter.