Is there an expression for the feeling of wishing you had met someone earlier?

REGRET / re·gret
rəˈɡret /

verb
1.
feel sad, repentant, or disappointed over (something that has happened or been done, especially a loss or missed opportunity).
"she immediately regretted her words"

synonyms: be sorry about, feel contrite about, feel remorse about/for, be remorseful about, rue, repent (of), feel repentant about, be regretful at/about

noun
regret; plural noun: regrets
1.
a feeling of sadness, repentance, or disappointment over something that has happened or been done.
"she expressed her regret at Virginia's death"

synonyms: remorse, sorrow, contrition, contriteness, repentance, penitence, guilt, compunction, remorsefulness, ruefulness. See Google regret

Regret, plain and simple. You regret you had not met them sooner. We're not supposed to regret our decisions in life because we're "Living in The Now" these days, but that has nothing to do with the meaning of the word.