Quoting Theoi,
In one, Ariadne, the daughter of King Minos of Krete, assisted Theseus in his quest to slay the Minotaur, and then fled with him aboard his ship. However, when they landed on the island of Naxos, Theseus abandoned her as she was sleeping.
Well, you don't just abandon a person who saved your life on an island these days.
Why? Is it just a characteristic of a selfish man? Was he going anywhere?
The answer depends on which account you read. As per Theoi:
One (less popular) account claims that Ariadne was killed by Artemis:
...but when they arrived in the island of Dia (Naxos), she was killed there by Artemis. (Hom. Od. xi. 324.) The words added in the Odyssey, Dionusou marturiêisin, are difficult to understand, unless we interpret them with Pherecydes by "on the denunciation of Dionysus," because he was indignant at the profanation of his grotto by the love of Theseus and Ariadne. In this case Ariadne was probably killed by Artemis at the moment she gave birth to her twin children, for she is said to have had two sons by Theseus, Oenopion and Staphylus.
Another account claims that she was abandoned by Theseus, either because Theseus was unfaithful and deceiving, or because Theseus was ordered to abandon her by Dionysus, who wanted to marry Ariadne himself.
The more common tradition, however, was, that Theseus left Ariadne in Naxos alive; but here the statements again differ, for some relate that he was forced by Dionysus to leave her (Diod. iv. 61, v. 51; Paus. i. 20. § 2, ix. 40. § 2, x. 29. § 2), and that in his grief he forgot to take down the black sail, which occasioned the death of his father. According to others, Theseus faithlessly forsook her in the island, and different motives are given for this act of faithlessness. (Plut. Thes. 20; Ov. Met. viii. 175, Heroid. 10 ; Hygin. Fab. 43.) According to this tradition, Ariadne put an end to her own life in despair, or was saved by Dionysus, who in amazement at her beauty made her his wife, raised her among the immortals, and placed the crown which he gave her at his marriage with her, among the stars. (Hesiod. Theog. 949; Ov. Met. l. c. ; Hygin. Poet. Astr. ii. 5.)
Ariadne was left by Theseus because he needed a new begining after disappointing his king, Ariadne's father. With her on his side, Theseus could not get that kind of peace he was looking for. So he left her, However this is full proof that theseus was absolutely a selfish man.
Mary Renault proposes another option - that Ariadne took part in the chthonic goddess rites of Naxos that still involved human sacrifice, and when Theseus saw this he was repulsed and realized that she would be what Medea was to his father Aegeus.
For reference check out The King Must Die and the Bull from the Sea.
