Can a person ever be too flexible?

Too flexible? It depends on what you mean by "flexible." it's good to be caring of others and generous with yourself, but you also need healthy self-interest -- so you can look out for your own needs & wants as well. Otherwise, you could end up feeling resentful of the other person you've been trying to be "flexible" with. Healthy relationships involve 2 people, both worthy of consideration and care.