For those who are wondering one reason why some believe hell to be eternal, consider the following from one of my favorite Catholic theologians to read, Joseph Pohle (it also shows the importance of the intercessory work of Christ and having a proper theology thereof):
As Christ ceased to
acquire merits after His death so a fortiori will man. (Joseph Pohle, Eschatology
or The Catholic Doctrine of the Last Things: A Dogmatic Treatise [St.
Louis: B. Herder Book Co., 1920; repr., Veritatis Splendor Publications, 2014],
23)
Our Lord performed no
meritorious actions (in the technical sense of the term) outside of the period
of His earthly pilgrimage (status viae). Hence the terminus ad quem was
the moment of His death (The question whether this limitation of Christ’s
meritorious action is based upon a positive and free decree of God, or whether
it is due to an intestine necessity, is purely speculative, and will be
discussed in Eschatology). That this is the teaching of Holy Scripture may be
gathered from such texts as John IX, 4 sq.; Heb IX, 12, X, 11 sqq. True, St.
Paul teaches that the glorified Redeemer continues to “make intercession for us
in Heaven” (Rom. VIII, 34; Heb. VII, 25). But the intercession He makes for us
in Heaven is based on the merits which He gained on earth and aims solely at
the application of these merits to individual men. (Joseph Pohle, Soteriology:
A Dogmatic Treatise on the Redemption [St. Louis: B. Herder Book Co.,
1914], 57)