What motivates children to start walking?
You might be interested in Alison Gopnik's book &quot;The Gardener and the Carpenter&quot;, where she gives thoughts on how children develop and learn. Children are very disinhibited in their behavior, and this is generally considered to be a consequence of their prefrontal cortex being immature, but she proposes that this is not a bug but a feature. That there is a tradeoff in any algorithm between &quot;exploiting&quot; the possibility space (incrementally improving your solution until it's as good as it can be) and &quot;exploring&quot; the possibility space (casting a wide net to find totally different solutions that might be even better). If you just &quot;exploit&quot; you are liable to stay stuck in local maxima; if you just &quot;explore&quot; you're never going to find a good solution because your search is too random and the possibility space is too large to examine every possibility. Good optimizing algorithms have alternations between the phases, with &quot;explore&quot; phases to look at many different areas of the possibility spaces, and &quot;exploit&quot; phases to optimize the most promising candidates. Gopnik's argument is that children are the &quot;explore&quot; phase of human development, and that is why they are disinhibited: they are motivated to try many different things for the sake of it, with no specific objective in mind or much thought to the consequences. (and in this view, an important job of parents is to make sure the consequences won't be fatal).
This might answer some of your question of babies thinking about &quot;pros and cons&quot;. It might be more realistic to see them as thinking only in terms of &quot;pros&quot;. Not, &quot;if I do this this will be the consequences, if I do that those will be the consequences, what should I do given the consequences I want&quot;, but &quot;what happens if I try this? And that? And that? And that? And that?&quot;
Another factor that wasn't mentioned in the comments is imitation; children like to imitate others around them and odds are they see adults walking all the time.
In terms of walking though I'd expect instinct to play the major role. Not that walking would be hardwired per se, but there are probably movements and reflexes that babies are hardwired to try, and once they get to a physical maturity where walking is possible the odds are that they will try those movements, and find the results interesting at least, profitable at best.