I'm not certain that it's an absolute, the idea of 'always' being strong and secure. Relationships involve two people. On any given day, either or both can have off days.
The need for reassurance however is something that can be looked at on its own. Symbolizes a level of dependency as a means of viewing your own self worth as dependent on his and your relationship. For instance, if the relationship crumbles, so would your sense of self and identity. That's not a fun place to be. It gives him, even if he doesn't want it, too much power over how you feel on a day to day, moment to moment basis. You are more than your relationship.
Ever heard the expression, Do your own thing, or I'll be me or I'll do me, you do you. That last one is urban slang.
There's dependence, independence and interdependence. The later, is the end goal in a relationship.
Find yourself outside of the relationship, your identity, that is and you'll find security. And in finding security, you may find what it is that is not only touching a wound ,but also what need is unmet.