The first time, Skyfire charitably thought it was an accident. Starscream looked contrite as he pulled the slivers of metal out of Skyfire's chest-plating, fingers caressing the painful, but not serious, gashes left behind.
The second time, this time having to have his arm mended, Skyfire reviewed the entire experiment in his head and decided that perhaps in the future it would be wise if he did the mixing of elements in their attempt to make a more refined energon. He was hesitant to suggest that perhaps Starscream was injudicious in doing so, but explosive results twice in a row were the beginning of a statistical set.
That the third time injured them both, because Starscream decided he was too slow in pouring the new liquified element from Lithone, was of little solace to Skyfire. He could not rein Starscream's impetuous nature in, and let Starscream be himself, so Skyfire undertook to gain a better way of protecting them both. It was an investment toward their future as explorers, Skyfire told himself as he underwent the long process of having heavier shield generators installed in his frame.
However, it was after the incident on their first scouting mission beyond Lithone's system that Skyfire realized Starscream's impetuous ways were call for more than just shields. He tended to be quite peaceful, but when the natives of their third planet-fall turned hostile at Starscream, over what his partner said was merely a misunderstanding, he wished that he had armaments capable of being an assist to the more war-designed Seeker. In mentioning this to Starscream, limping their way back to Lithone, for it was closer than Cybertron, the Seeker's optics glowed brightly, and Skyfire found that his partner had plenty of ideas on how and would hear of no others doing the work.
In a future far from this, Starscream had reason to regret that choice, when Skyfire consistently proved able to beat he and his trine-mates back or at least to a stalemate.