At the beginning of the play, in act 1 scene 1, he refers to Demetrius's past love for Helena saying “These antique fables nor these fairy toys... I will believe, as soon as they persuade me that Demetrius doth dote upon your eyes (Hermia)." This suggests that Theseus doesn't give much credence to such fanciful romances and he finds it hard to believe in Demetrius's sudden change of heart towards Helena.
However, Theseus' opinion changes throughout the play, particularly after the performance of "Pyramus and Thisbe," which is presented as entertainment during his wedding celebration. Watching the play-within-a-play seems to impact his thoughts on love and romantic entanglements.
In particular, he remarks in act 5 scene 1:"The best in this kind are but shadows; and the worst are no worse if imagination amend them". He acknowledges love is not always rational and can create illusions and make the impossible seem tangible , This recognition shows Theseus' willingness to reconsider his perspective on the romantic pursuits of youth
Additionally, Demetrius' unwavering devotion to Helena at the end of the play seems to further soften Theseus' opinion. Recognizing the importance of following one's heart regardless of external circumstances Theseus eventually gives Hermia and Lysander his blessing in Act 4 Scene 1 stating I'll follow you, and lead you all the way; Through briers and thistles; through rocks and stones I'll be a bridge that shall convey all over
Overall, while Theses's initially dismisses the lover's adventures as irrational he becomes more reflective. After witnessing how their trials and entanglements have ultimately resulted in deeper emotions and true commitment, Theseus becomes supportive of their decisions even though there still remains a touch of scepticism about the true nature of love and its irrationalities