Charles V getting the spanish titles was luck. The amount of tragedies that needed to happen for him to hit the jackpot was staggering. Let's explain:

Queen Isabel of Castile was married to king Ferdinand of Aragon (who also had Naples, Sicily, and Sardinia). They had a number of sons and daughters that would have priority in the matters of inheriting the crowns.

- Prince Juan, AKA the prince who died of love. He was married to Margaret of Austria in 1497. The poor guy died after only six months of marriage, leaving his wife pregnant. The mourning and depression lead to a miscarriage.

- Infanta Isabel of Aragón and Castile, queen consort of Portugal. She married Manuel I of Portugal in 1497. She fell pregnant, and died giving birth to a son in 1498:

-- Miguel de la Paz, crown prince of Portugal, prince of Asturias, and Gerona. Died at two years of age.

All of these would have been earlier in line than Charles V for getting the Spanish realms. Furthermore, Ferdinand of Aragon married Germaine of Foix in the hopes of getting Aragon out of Philip the Handsome's hands knowing that his daughter Joanna was not in good mental health. Of this second marriage came a son:

- Juan of Aragon and Foix, born in 1509, lived for two hours.

Coincidence might have been the main factor in the accumulation of titles by Charles on the Spanish side of things, although I do not entirely agee with the assesement of u/tywindevillena. The marriages of Philip the Handsome to Juana of Casitile, and that of his sister, Margaret of Austria to Juana’s brother Juan, were, in my view, entirely made with the intention that someone with Burgundian-Habsburg blood would inherit the throne of Castile. The rest of it can of course be ascribed to dynastic coincidence: nobody counted on all the deaths that would eventually lead Philip the Handsome to inherit the Castilian Kingdom through his marriage to Juana. In fact, the Burgundians were not very popular in Spain, at least not during the short reign of Philip the Handsome. It is also true that before Charles, Phillip’s eldest son and heir, many other princes had better claims on the Kingdom of Aragon, all of whom died before they became of age or were able to produce an heir of their own. Yet by the time Charles was declared of age in 1515, he was the sole heir to both Spanish thrones.

Then, there were the German lands he inherited: Charles grandfather, Maximilian of Austria, was the archduke of Austria. When his son, Philip the Handsome died unexpectedly in 1506, Philip’s son Charles became his grandfather’s heir, and as such, future archduke of Austria, the homeland of the Habsburg family. Like his father before him, Maximilian was also elected Emperor of the Holy Roman Empire (largely what is now Germany and Austria). This title was not yet hereditary in the sixteenth century, but through lobby-work and bribery, the Habsburgs managed to have a member of their family elected Emperor until the eighteenth century. This tradition started with Charles, who became Emperor in 1519.

Finally, there were the Burgundian lands, which, by the sixteenth century, were a combination of different independent principalities, all of which were more or less under control of the Duke of Burgundy, in the sense that the Duke of Burgundy was also the Count of Flanders, as well as the Duke of Brabant etc. Here too, we have to go back to Maximilian of Habsburg, Charles’ grandfather. He had been married to Mary of Burgundy, the sole heiress of the Duke of Burgundy, Charles the Bold. The Duke Charles the Bold had always wanted the title of King of Burgundy, and negotiated with Maximilian’s father, the Emperor of the Holy Roman Empire, to get this title. The Emperor, after all, was the only one who could grant the title of King. One of the conditions to the deal, was that Charles’ daughter Mary would marry the Emperor’s son, Maximilian. This marriage did happen, although not under the conditions Charles the Bold had wanted and he never became King (I will not go into detail about this here, that is a story for another day). The son of Mary and Maximilian was Philip the Handsome and his eldest son was the Charles of your question. When his father died, this Charles thus became Duke of Burgundy, with all the other titles that came with that title.

So to answer your question, yes, there was a lot of coincidence involved in this accumulation of titles, but this coincidence could only happen under the influence of the meticulous dynastic planning of the Burgundian-Habsburg family. If you would like to read a good recent biography of Charles V, I would recommend the book “Emperor, a new life of Charles V”, by Geoffrey Parker.