While Alexander had been the only healthy male child, he hadn't been the only son of Philip II of Macedonia. According to John Maxwell O'Brien, Olympias may have caused brain damage to her step-child, Philip's other son, Philip Arrhidaeus.
Alexander had never considered his half-brother a threat until his father and Pixodarus of Caria (part of the Persian Empire) began to discuss marriage plans between their children. Philip Arrhidaeus was to marry Pixodarus' daughter.
Alexander, persuaded by his friends or mother that the marriage was more important than Philip let on -- that it signified Alexander's replacement by his half-brother -- sent an emissary to Pixodarus countermanding the king's arrangement. The emissary insisted that not Arrhidaeus but Alexander be the bridegroom.
This was more than disobeying a parent.
When Philip found out, he was enraged, banished four of Alexander's closest friends and ordered Alexander's emissary brought home in chains.
In the end, Pixodarus decided against the alliance with Macedonia and sided, instead, with the Persian Darius III.
" ... for when Pixodorus, viceroy of Caria, sent Aristocritus to treat for a match between his eldest daughter and Philip's son, Arrhidaeus, hoping by this alliance to secure his assistance upon occasion, Alexander's mother, and some who pretended to be his friends, presently filled his head with tales and calumnies, as if Philip, by a splendid marriage and important alliance, were preparing the way for settling the kingdom upon Arrhidaeus. In alarm at this, he despatched Thessalus, the tragic actor, into Caria, to dispose Pixodorus to slight Arrhidaeus, both illegitimate and a fool, and rather to accept of himself for his son-in-law. This proposition was much more agreeable to Pixodorus than the former. But Philip, as soon as he was made acquainted with this transaction, went to his son's apartment, taking with him Philotas, the son of Parmenio, one of Alexander's intimate friends and companions, and there reproved him severely, and reproached him bitterly, that he should be so degenerate, and unworthy of the power he was to leave him, as to desire the alliance of a mean Carian, who was at best but the slave of a barbarous prince."
Plutarch Life of Alexander
